Frodo Baggins, the main character of The Lord of the Rings, becomes its unwitting hero when he's adopted by Bilbo Baggins (his cousin, but known affectionately as "uncle"). Frodo is an only child and an orphan — like Tolkien himself — who comes to live with Bilbo at Bag End at the tender age of 21 (hobbits don't come of age until they reach 33). After Bilbo's surprise exit from his 111th birthday party — which happens also to be Frodo's 33rd birthday — Frodo inherits his entire estate, along with his magic gold ring, which Gandalf barely persuades Bilbo to leave behind. The ring is the Ring, of course, and Frodo is saddled with the responsibility of taking it to its destruction in Mordor.
Frodo is, then, a "drafted" hero. Even Bilbo, who goes kicking and screaming on his adventure with the Dwarves, does it for the adventure itself and its potential reward. Frodo, however, begins his quest much more out of necessity — it's brought upon him as necessary to save the Shire that he loves so well. A hero involuntarily burdened with a quest is the farthest thing from the knight errant looking for dragons to slay. Although at times Frodo hankers for adventure — he wants very much to go with Bilbo when he leaves the Shire — Tolkien is also careful to show Frodo's reluctance to be the savior of the Shire when Gandalf reveals to him the true identity of the Ring and its danger.
Tolkien in no way presents Frodo as a young headstrong hero aching for a fight, but rather as a reluctant one full of trepidation and self-doubt. Frodo seems to me more of a modern-day hero — who does his duty for lack of another choice — than a typical fantasy or legendary hero, who does it for the challenge and test of his prowess. This is part of what makes The Lord of the Rings so very different from other fantasy tales. If a typical fantasy writer had written the tale, he or she would undoubtedly have made Aragorn and not Frodo the main hero who confronts and destroys Sauron (as Aragorn's ancestor Isildur did before him). With Frodo, Tolkien does his own take on confronting evil and gives us a more accessible model for our own heroics.
Frodo's courage and internal struggle
Throughout his quest to take the Ring to Mordor and destroy it, Frodo faces an internal struggle with the Ring's evil and his own self-doubt. A pivotal point occurs at the Council of Elrond when, to his own surprise, he finds himself volunteering with his famous expression, "I will take the Ring, though I do not know the way."
During the journey, Frodo many times manifests a courage that he doesn't know he has. He shows courage at Weathertop where, instead of being paralyzed with fear, he stabs the Lord of the Ringwraiths. Later, after crossing the ford of Bruinen at the border of Rivendell, he defies the Black Riders who've come to take him back to Mordor. The problem is that Frodo's first impulse whenever he needs to summon his courage is to put on the Ring and use its power. He actually does put the Ring on prior to confronting the Ringwraiths at Weathertop and stabbing the Lord of the Nazgûl. He also resorts to putting it on when Boromir tries to take it from him at Amon Hen.
Frodo first becomes aware of the Eye of Sauron searching him out when he's sitting on the Seat of Seeing at Amon Hen wearing the Ring. In this encounter, you begin to see the seriousness of Frodo's internal struggle, for he is not sure whether his response to Sauron is "never in your wildest dreams" or "Chief, I'm on my way." When he finally realizes that he must take off the Ring or risk Sauron getting a fix on his location, he still wrestles with the decision to take it off.
In a very telling description, Frodo is said to be "writhing" during this struggle. Suddenly, Tolkien tells us, Frodo becomes aware of himself again and takes off the Ring in the nick of time. This is where Frodo makes the decision to go it alone to Mordor, saying, "I will do now what I must." Of course, Sam won't let him go alone. As he perseveres toward Mordor, you see Frodo morphing from simple Ring-bearer into wannabe Lord of the Ring. As he and Sam get closer to Mount Doom, the seat of the Ring's power, its influence weighs heavier and heavier on Frodo, and he becomes increasingly possessive of it (that old "My Precious" thing). Despite the terrible internal struggle, Frodo finds the courage and the stamina to continue on to the bitter end.
Frodo's Christ-like suffering
It doesn't take a biblical scholar to feel some similarity between Frodo's struggle to carry the Ring up Mount Doom and Christ's struggle to carry his cross to Calvary. By the time Frodo reaches Mount Doom, he is so weighed down by the power of the Ring and despair over its destruction that Sam carries him and the Ring up the path to the Crack of Doom — shades of Simon the Cyrenian bearing Jesus' cross to Golgotha.
Any parallel, intentional or not, between Frodo and Christ ends when Gollum attacks Frodo on the path in their second-to-last encounter. Gollum's effort to wrest the Ring from him re-ignites Frodo's will, showing how stern and powerful he has become under the Ring's influence. In prophetic and commanding words, Frodo fends off Gollum, warning that if he ever touches him again, he will be cast into the Fire of Doom.
Unlike Christ, who at the height of his trial on the cross submits his will to God's and commends his spirit into His hands, Frodo, at the climax of his ordeal with the Ring, exerts his own will first by choosing not to complete the quest, saying, "I will not do this deed." With this declaration of will, Frodo claims the One Ring as his own and puts it on to openly reveal himself to the Eye of Sauron.
One can only surmise that at that point Frodo is prepared to directly challenge the Dark Lord for the title of Lord of the Ring. The idea that Frodo could best Sauron in a contest of evil, even wearing the Ring, is hard to believe. More likely than not, the Ring is simply using Frodo to get back to its master by revealing its whereabouts.
The great irony of this situation is that the moment Frodo feels as though he's finally mastered the Ring by claiming it and deciding against its destruction is precisely when the Ring takes completely mastery of him and turns his will into its will. Frodo's failure at Mount Doom is the polar opposite of Christ's victory on the cross, wherein Christ masters his suffering and death by submitting his will to this fate.
Frodo as the failed hero
Frodo's failure at Mount Doom is unlike anything found in folklore. Although mythology is full of tragic heroes undone by hubris after achieving greatness (such as Oedipus and Jason of the Argonauts) and heroes who are ultimately defeated in their final quests (Beowulf and King Arthur), no other hero, just at the point of achieving the quest, turns against it and abandons it.
This begs the question of whether Frodo can even be considered a hero. But he is a hobbit of great courage and stamina despite his collapse at the end, for a hero of lesser character and courage would not have made it as far as he did without succumbing to the Ring's power. The cause of Frodo's breakdown may not be a lack of will but rather an overabundance of it (the will amplified and used by the evil of the Ring).
One can only speculate on why Frodo fails right when he's poised to succeed. It may have to do with Tolkien's idea of providence and eucastrophe and the idea that none of us can succeed alone — especially not through the power of our will alone.
Frodo as the wounded hero
If you consider the One Ring as some kind of weird psychic amplifier that boosts your willpower and turns it to its own, then Frodo does extraordinarily well in reaching the Crack of Doom — one can only imagine what the Ring would have done in the hands of more strong-willed characters such as Galadriel, Aragorn, Gandalf, or Boromir.
What's more, the ordeal of the Ring haunts Frodo long after he returns to the Shire. And there, instead of being received as a hero, he's actually shunned in favor of his more boisterous companions Merry and Pippin, who are welcomed back as natural leaders. Although he is instrumental in saving the Shire from Sauron's domination, Frodo is never really able to be comfortable there again.
In this way, Frodo is a lot like war heroes who never become fully reintegrated on returning home and are ever after haunted by their war service. Tolkien certainly knew something about this, given that he was in the trenches of France during the First World War. He also undoubtedly encountered many such heroes after the Second World War, years during which he worked diligently on the writing of The Lord of the Rings.
Even as the other hobbits in the Fellowship find their places in the reconstructed Shire, Frodo remains alienated and often plagued by illnesses — which recur on the anniversaries of his encounter with the Ringwraith on Weathertop and his failure to destroy the Ring. Finally, upon completing his memoirs — the supposed source of The Lord of the Rings — he departs Middle-earth forever upon an Elven white ship, headed for the Blessed Realm.