Frodo is the nephew of Bilbo Baggins and the heir of his estate. Like Bilbo, Frodo is educated in lore, has been known to talk with Elves, and is fairly well-to-do. Like Bilbo, he seems to have a small, but real, desire for adventure, uncommon among hobbits. In a letter to English publisher Rayner Unwin dated December 15, 1965, Tolkien writes that The Hobbit and The Lord of the Rings are about "the achievements of specially graced and gifted individuals." When Bilbo gifts him the Ring, Frodo inherits much more than the ability to become invisible. Gandalf convinces him that keeping the Ring endangers the Shire—not to mention the entire world. Although he is conflicted about leaving the Shire, Frodo follows through because he knows it is the right course of action. This decision—to step foot on the Road that "goes ever on and on"—will lead to many more choices, many more decisions. Frodo meets them each with the same determination and sense of inevitability. He can see he must take the Ring to Mordor, and so he does. Even when it looks like he might not return from his quest, he presses on. Even when the Ring becomes heavier and heavier, and its influence stronger, he presses on. The heroism of Frodo is the heroism of determination and steadfast adherence to doing what is right.
Like Frodo, Sam is a hobbit from the Shire, and also has a bit of longing to become more than an ordinary hobbit—shown by his desire to meet Elves—but his character arc is less steady. While Sam's loyalty to Frodo is unchanging, he rises from simple gardener to hero without the benefit of any of Frodo's privilege. Yet his determination to see the quest through to the very end eventually matches Frodo's. According to Tolkien, Sam was, in some ways, more of a "main character" than Frodo, because he experiences the most character growth. In a letter to his son Christopher Tolkien dated December 24, 1944, Tolkien writes, "Sam is the most closely drawn character, the successor to Bilbo of the first book, the genuine hobbit. Frodo is not so interesting, because he has to be high-minded, and has, as it were, a vocation." Sam plays an important role in representing the ordinary world of simple peace and pleasures, which make fighting the war worthwhile, especially considering his marriage and family. In a letter to editor Milton Waldman, Tolkien claimed the moral of the story "is that without the high and noble the simple and vulgar is utterly mean; and without the simple and ordinary the noble and heroic is meaningless."
Gandalf the Grey is one of five wizards (called Istari in some of Tolkien's writings) sent to Middle-earth to help its inhabitants fight Sauron. These wizards take on the look of old men but are actually powerful beings who have insight and knowledge far beyond that of humans. From Frodo's perspective, Gandalf comes and goes randomly and without warning, but in reality he is constantly working at his task. Without Gandalf, resistance against Sauron would have been far too weak to succeed. Certainly in The Lord of the Rings, Gandalf is both familiar and kindly (creating fireworks for Bilbo's birthday) as well as distant and powerful (as he interacts with the kings of Rohan and Gondor, for example, or when he fights the Balrog).
When the hobbits first meet Aragorn, they know him by the name others in Bree call him—Strider. Soon, though, it becomes clear Strider is much more than a vagabond or wanderer. He is Aragorn, son of Arathorn and the heir of Isildur, who cut the One Ring from the hand of Sauron. This means Aragorn is the heir to the throne of Gondor, of which Denethor is Steward. In some ways, he seems destined to become king, and many of his decisions are made because he feels the time is right or a certain course is necessary. He fulfills prophecies and ultimately does become king. However, Aragorn's fate is far from certain from within the story—Sauron's forces are overwhelming and the fight against him is always on the brink of failure. One important aspect of Aragorn's character is his role as a healer. He heals the wounds of his traveling companions from the outset, and after the Battle of the Pelennor fields, he heals those who have become ill with the "Black Breath"—a malady that comes from fighting the Ringwraiths. This proves to be essential to his eventual coronation, since Gondorian lore says, "the hands of the king are the hands of a healer."
Sauron is the main villain of The Lord of the Rings and the creator and owner of the One Ring. Yet he is not the original evil in Tolkien's mythos—just one of its most powerful servants. In the First Age, he was the main servant of Morgoth (Melkor). In the Second Age, he taught the Elves how to make rings of power, while secretly crafting the One Ring to be the master of the other rings and so ensnare those who wear them; he also influenced the Men of Númenor, leading to the fall of Númenor. In the Third Age, he quietly built up his strength as the Necromancer, only to emerge as ruler of Mordor and master of the Nazgûl.
Gollum is the character from The Hobbit from whom Bilbo obtained the Ring. Part of a people similar to hobbits, he gained the Ring by murder and lies, and so was more vulnerable to its corrupting influence. In The Lord of the Rings, Gollum is the case study of what could happen to a hobbit completely overtaken by the evil of the Ring. He shows what could have happened to Bilbo or Frodo if they had taken the Ring by force or had used it with intent to further their own power.