It is difficult to name the hero of The Lord of the Rings, as there are so many throughout the tale—just how does one pick the hero from the likes of Frodo Baggins, Samwise Gamgee, Aragorn son of Arathorn, and Gandalf the Grey? Then there are Meriadoc Brandybuck, Peregrin Took, Éomer son of Éomund, Théoden King, and, of course, Boromir and Faramir sons of Denethor to capture the attention of all, not to mention Gimli and Legolas, Elrond of Imladris, and both Galadriel of Lothlórien and Éowyn of Rohan—plus the subtle presence of Arwen Undómiel.
What a rich mine of characters to examine, and such a number of heroes and heroines to pick from!
There is no question, however, that the two characters about whom the tale tells most are Frodo Baggins of Bag End in the Shire and Aragorn son of Arathorn with all his various names and titles. And how many parallels there are between the two of them!
Aragorn lost his father while but a toddler, being two years old when Arathorn took an arrow to the eye and died probably instantly.  At that point the child and his mother were spirited away to Rivendell where Aragorn began his fostering in Elrond’s home at a much earlier point of time of life than was ordinarily experienced by the heirs of Isildur, coming to think of Lord Elrond as his father until he was judged a man grown at the age of twenty. It was likely he saw few enough children near his own age during those years to appreciate what it meant to play as do other children, or what childhood would mean to other mortals. His life was, in many ways, extremely protected and somewhat artificial. It is likely that besides his mother only a very few trusted others from among the Dúnedain of Eriador would have been aware of his existence, for his continued safety lay in keeping the secret of his survival to maturity.
Frodo Baggins lost not only his father, but his mother, too, and at a time in his life that left him both best prepared for adapting to whatever new life he might know and yet leaving him particularly vulnerable to many influences. We know little of his earliest life, for we are not told where within the Shire he was born or lived before his parents’ deaths, nor how well his immediate family got along with Bilbo Baggins, the Brandybucks of Brandy Hall, or the Tooks either of the Great Smials or those who farmed the lands near Whitwell. A Hobbit child of eleven to twelve would be roughly the equivalent of a Mannish child of seven to eight in many ways; and such children tend to adapt fairly well to new circumstances when they are well treated, and yet are of an age to keep memories of their birth-parents and earlier life. It is likely that his new guardians would consider him a good deal more fragile than he proved, but that they could well miss important clues indicating where his true emotional hurts had been taken.
Drogo and Primula Baggins do not appear to have resided primarily at Brandy Hall, but do appear to have been frequent visitors, reportedly due to Primula’s father spreading a fine table for residents and guests. We know they went out on the river one evening, but the circumstances of why they did so have not been divulged to us. All we know is that they both drowned that night, leaving their only living child Frodo in the physical custody of Primula’s Brandybuck family. And there the child remained for nearly ten years, until the year Bilbo took custody of his young first and second cousin, once removed each way, and adopted him as his heir, reportedly finding Frodo the one younger relative he had who displayed any spirit. 
So, here is a parallel between these two characters: both were torn from their original home by the loss of parents, and raised in situations to which they were not necessarily inclined by nature. But where Aragorn, having come there at the age of two, came to see Rivendell as his primary home and the Elves now surrounding him as part of his ordinary experience, Frodo had to know that Brandy Hall was not his home, and had to adjust his thinking to accommodate the fact that the Brandybucks had gone from benevolent relatives with whom he’d been accustomed to visit temporarily to his primary caretakers. Whatever happened during Frodo’s childhood from the time of his parents’ deaths, he appears to have felt displaced until he returned to the Shire proper to live in Bag End with his “Uncle” Bilbo, at which point he found himself sharing Aragorn’s unusually Elven education.
Aragorn, after all, as “Estel” was also living as the ward of a nominal “uncle,” for Elrond Peredhel was, after all, brother to the founder of Aragorn’s own line, Elros Tar-Minyatur.  As Elrond’s foster son, young Estel was undoubtedly given a thorough education in the histories of his mixed lineage, and was sufficiently familiar with the Lay of Leithien and particularly the portions dealing with the meeting of Lúthien and Beren that he appears to have sung it frequently—he was singing it when he first saw Arwen Undómiel, and sang it again for the Hobbits during their stay in the dell below Weathertop. [22, 8] He was a lore-master from the start, well versed in the tales of the First and Second Ages, and fully educated in the history of evil as expressed by Melkor and Sauron and their various machinations.
Bilbo Baggins, apparently following in the footsteps of his remarkable maternal grandfather, old Gerontius Took [1, 24], had developed an appreciation for the history of the greater world, an appreciation that was undoubtedly increased by his adventure to the Lonely Mountain and back. It is most likely that on his return he wanted to learn more, and that he looked to both Gandalf and Elrond to satisfy his curiosity and thirst for more information. [1, 2] That part of the reason he wished to take Frodo as his ward and protégé was so as to foster similar interests in the younger Hobbit is very likely. So it is also likely that Frodo was also interested in all things Elvish when young, and had this appetite both encouraged and filled by his new guardian. In this way it is likely that Bilbo had given Frodo as thorough an education in the history both of the Shire and of the world Outside as it was possible to give a Hobbit of the Shire, preparing him for his own journey in a unique manner. It is likely that Frodo saw much of this information he learned as being purely academic until he found himself in the midst of his own adventure, at which time it became far more important to realize these were not just romantic tales he’d heard, but the histories that led up to his own situation, an appreciation shared in the end with Samwise Gamgee, who was to state this openly as he and Frodo approached Mordor.
For it was Sam who recognized aloud that the light held within the Phial of Galadriel gifted to his Master was the same as that borne aloft by Eärendil the Mariner, and taken by Beren One-hand from the Iron Crown of Morgoth in Thangorodrim, thus making his and Frodo’s quest but a continuation of the older stories. That he should see this with a level of wonder while Frodo saw it in a more ironic and fatalistic light was, I must suppose, to be expected. 
Frodo had also probably seen an Elf or two while in Bilbo’s company, most likely messengers from Rivendell. Bilbo himself appears to have come into personal contact with the Elves of the wandering companies as well, for Gildor Inglorion discusses him as if he is well familiar with the older Hobbit, but speaks of knowing Frodo only from having observed him in Bilbo’s company on jaunts about the Shire. Frodo had mastered the appropriate Elvish greetings, and appreciated that the song they heard from Gildor’s folk as they approached was about Elbereth Gilthoniel, the Valië who gave the universe its stars.  It is therefore likely that Frodo had studied some Elvish, and most likely Sindarin, as taught him by Bilbo, and that these studies from his youth became more important to him as his own journey brought him into physical contact with the Elves of Rivendell and Lothlórien, perhaps somewhat enhanced by the companionship of Legolas during the quest.
Aragorn, on the other hand, had been raised probably to be multilingual. He would undoubtedly have spoken Sindarin as his primary language, but nearly equally with Westron and Adúnaic, and he would probably have also learned to be fluent even in the high tongue of Quenya, as it would be expected and needful for the one who sought to combine the rule of the two Númenorian realms founded by Elendil and his sons.
Here, then, would be another parallel between Frodo and Aragorn—sharing an unusually broad education for each one’s kind, including the study of Elven languages.
Both would also have been taught administration and record keeping. Aragorn, after all, was being groomed to become Chieftain of his folk as well as hopefully King of a reunited Arnor and Gondor.  Frodo, as the expected next Baggins Family Head, would also have been required to learn how to administer and keep track of family finances and assets as well as those of the Master of Bag End, and would have had to track the history and status of his now dwindling family of name. There were few enough Bagginses left when Bilbo quitted the Shire; how many might remain at the time Frodo followed him seventeen years later is questionable, as it appears that Frodo and Lotho were the only sons to remain to carry on the family name within the Hobbiton region at the time. Unless someone within the family broke off from the main Baggins line and set up a small enclave elsewhere within the Shire, it appears that Frodo marked the end of a most fascinating lineage. 
Both were in the middle years of their expected lifespan at the time they met, and both appeared markedly younger than their true ages, Aragorn due to his nearly pure Dúnedain heritage, and Frodo due to the prolonging of apparent youth and vitality offered by the Ring he carried within his pocket. Aragorn thus appeared not appreciably older than Boromir, Éomer, and Faramir; Frodo appeared a contemporary of his actually younger Hobbit companions; yet their true contemporaries among those taking part in the quest were each other.
Both came under the tutelage and guidance of Gandalf the Grey, who did his best to ensure that Aragorn be prepared for his future role by learning the ways of his future allies and enemies, and who promised to keep two eyes on Frodo, as often as he could spare them. And where most of the Hobbits of the Shire saw Gandalf as a dangerous instigator of adventures, [1, 2] Frodo felt comfortable enough with him to appreciate that the knowledge shared with him about the origins of the Ring and Gollum was trustworthy and to be respected and acted upon. 
One of the most fascinating parallels between the two is the closeness each developed with others, and the protective nature of the friendships they evoked. None within Rivendell appears to have questioned the surprisingly close bonds formed between their lord and his last fosterling from his brother’s descendants, but appear to have developed a keen love for Aragorn/Estel as well on their own parts.  Then, on his return to his own folk Aragorn appears to have quickly won the affection and fealty of his kinsmen, starting with Halbarad, who may well have been a first or second cousin, probably on Gilraen’s side of the family.  This gathering of fealty continued apparently throughout his life, and certainly led others to be drawn to him as was true of Éomer and Faramir from their first encounters with him. [13, 17]
It appears similar with Frodo—he had a handful of very close friends, mostly his younger cousins: Meriadoc Brandybuck, who undoubtedly thought of Frodo as next best thing to his own older brother considering how he’d have lived his first few years in Frodo’s company; Peregrin Took, who seems to have had such a relationship with Merry as Merry had known with Frodo and who saw Frodo as one he had the responsibility of protecting; Fredegar Bolger; and Folco Boffin—and, of course, Samwise Gamgee.  Pippin describes Sam as being willing to sacrifice himself on Frodo’s behalf, a description that certainly proved accurate!  But perhaps the most remarkable indication of Frodo’s particular charisma is evidenced by the fact that two more found themselves falling under his personal spell and naming themselves willing to spend themselves on his behalf—Faramir son of Denethor when he indicated that he would accept his life being forfeit for allowing Frodo to travel further through Ithilien, and Aragorn son of Arathorn: “…and if, by life or death I can save you, I will.” 
That he who was born to rule Gondor and Arnor reunited should himself vow fealty to Frodo Baggins, and before he’d had a chance to see Frodo prove himself in any notable way, speaks volumes about the characters of both. Like honors like. And to leap so from worrying about Frodo’s discretion to making the vow to protect Frodo at any cost speaks to Aragorn’s own personal integrity and discernment.
In return, Frodo responds by almost immediately accepting this scurvy looking wanderer who has accosted his party at the Prancing Pony, telling his Hobbit companions that he expects that the true servants of the Enemy would look fairer and feel fouler, a mixed compliment as Strider notes wryly. 
Aragorn is among the first to accept a place in the company of the Nine Walkers as one of Frodo’s companions, alongside Sam. From the beginning he is the one Gandalf consults as to which alternative route to choose, as well as the natural one to take over leadership of the Fellowship once Gandalf has fallen with the Balrog. He is the one negotiates with Haldir and his brothers regarding their entrance into Lothlórien, and who suggests the compromise by which all of their remaining party enter the Golden Wood blindfolded. And on Cerin Amroth Frodo first experiences a vision that echoes a previous visit by Aragorn to the place, when he seems to see the Man garbed as an Elven Prince as he’d appeared to Arwen when she first accepted him as the one who stirred her heart and plighted her troth to him; and here he also finds that a portion of his own heart remains as well—two shadowy ghosts to haunt the hill upon which Amroth once dwelt, the memories of elvish Man and elvish Hobbit remaining there indefinitely even though their corporeal selves must go on and will never return. 
The nobility of the Man is echoed ever in the Hobbit he has sworn to protect. Aragorn absolves the dying Boromir of the sin of seeking to take the Ring from Frodo ; as he and Sam take refuge on the hill of ash at the foot of Oródruin once the Ring has been destroyed, Frodo bids Sam forgive Gollum, as without the creature’s intervention the quest could not have been fulfilled.  Aragorn and Frodo both inspire others to freely offer up their lives for the good of all, and do so by their own examples. Both see all too clearly where their paths may well lead, and yet go on through the darkness, knowing that even if they fail they must at least make the attempt.
And there is the mysterious Light of Being that is discernible in each of them. Frodo realizes that Aragorn is the tall kingly Man he has envisioned before, seeming to see the Star of Elendil upon his brow , offering to return to him as Isildur’s heir the Ring at the Council of Elrond, and clearly seeing the aura of Aragorn’s nature as the Sacred King, the Healer King and the Renewer, about him at the Man’s coronation.  Gandalf, Elrond, and Sam all see the growing Light within Frodo Baggins, with Gandalf pondering on how, in time, Frodo will likely become as a clear vessel filled with light for eyes to see that can  as the changes to his nature apparently begun by the wounding by the Morgul knife but purified by the healing offered by Elrond continue over time. Sam also discerns a clear light emanating from Frodo on the Stairs of Cirith Ungol that is already apparently familiar to him ; and so recognizes that Frodo now belongs to the world of the Elves that when he realizes that Frodo is planning to leave the Shire he accepts the apparent decision of Frodo to retire to Rivendell with no question, then finds the grief of realizing he would not see Frodo again for many years if at all a blessed thing as he accepts Frodo’s invitation to ride with him to the Havens. 
Both Frodo and Aragorn have been given much knowledge by their mentors, knowledge that through their arduous experiences is transmuted into ineffable wisdom and that in the end deserves great reward. Aragorn’s reward is given him in this world—marriage to the one woman he has ever known who stirred his heart, the last of the Queens of the Elves remaining within the Mortal Lands, honor and responsibility in measure with the faithfulness, trust, wisdom, and strength he has ever displayed, and a true happily-for-ever-after life. But if Aragorn is the Healer King, Frodo Baggins is the Wounded King who accepted the wounds that do not heal, and by whose suffering the rest of the world is freed from pain.  Not for him bliss within this reality: to him is given the right to leave this world and find elsewhere the healing and fulfillment (or at least acceptance) of his state that cannot come to him here in the Mortal Lands. His reward lies elsewhere. He leaves yet a virginal character, ready to be purified and returned to his former blemish-free state, the acceptable sacrifice who was ready to die for the safety of all but who now must learn to live to that purpose instead. And if Aragorn is the regnant for Middle Earth’s new Golden Age, Tolkien himself described Frodo as Middle Earth’s Once and Future King, leaving to the hidden realms so as to return at need sometime in the future should Middle Earth ever need him to return to its further salvation. 
There is a somewhat skewed symmetry to the lives of these two: Aragorn began as a single mortal child living amidst the immortal Elves of Imladris and being nurtured by them; Frodo Baggins ends his years as the sole mortal being dwelling on Tol Eressëa amidst the immortal Elves of the Undying Lands, where he hopefully knows their healing and acceptance—at least from whatever time Bilbo has died until the coming of Samwise. During his years as Strider Aragorn knew derision and suspicion from the very folk the Dúnedain he led sought to protect in secret ; after his return the only people throughout Middle Earth who did not offer Frodo the respect and honor he’d earned were his own folk within the Shire.  Each, faced with the needs of the world in which he lived, offered up all he was capable of being; each received honor and love in keeping with their sacrifices and labors; and each must have found within the other a true brother of the heart and spirit. Aragorn could not have come to either his throne or marriage to his Queen without Frodo’s efforts to fulfill the quest; Frodo could not have come to his reward without the Queen’s gift to him of her own place on her father’s ship. 
Is it any surprise that I like to imagine that at one time it had been planned that these two should have been fraternal twin brothers, and with that plan marred by the epidemics of diseases intended to spark deaths and miscarriages set loose by Sauron, instead Frodo was allowed to be born among the one people within Middle Earth the dark Maia had always overlooked? Ah, how we appreciate now the sense of irony that Eru appears to have ever displayed!
1. The Hobbit: “An Unexpected Party”
2. Fellowship of the Ring: “A Long Expected Party”
3. Fellowship of the Ring: “Shadows of the Past”
4. Fellowship of the Ring: “Three is Company”
5. Fellowship of the Ring: “A Conspiracy Unmasked”
6. Fellowship of the Ring: “Fog on the Barrowdowns”
7. Fellowship of the Ring: “Strider”
8. Fellowship of the Ring: “A Knife in the Dark”
9. Fellowship of the Ring: “Flight to the Ford”
10. Fellowship of the Ring: “Many Meetings”
11. Fellowship of the Ring: “Lothlorien”
12. The Two Towers: “The Departure of Boromir”
13. The Two Towers: “The Riders of Rohan”
14. The Two Towers: “The Stairs of Cirith Ungol
15. Return of the King: “The Passing of the Grey Company”
16. Return of the King: “Mount Doom”
17. Return of the King: “The Houses of Healing”
18. Return of the King: “The Steward and the King”
19. Return of the King: “Many Partings”
20. Return of the King: “The Grey Havens”
21. Return of the King: Appendix A: “The Kings of Numenor”
22. Return of the King: Appendix A: “The Tale of Aragorn and Arwen”
23. Return of the King: Appendix C: The Family Trees: “Baggins of Hobbiton”
24. Return of the King: Appendix C: The Family Trees: “Took of the Great Smials”
25. The History of the War of the Rings: The End of the Third Age: “The Epilogue”
26. For a better understanding of the roles of the Sacred King and the Wounded King, see the collected works of Joseph Campbell.