The Fellowship of the Ring, and the Lord of the Rings trilogy as a whole, is an adventure story in which Frodo selflessly embarks on a hero’s journey to save Middle-earth. If Frodo represents the epitome of good, then he needs a foil to vanquish, one who represents the epitome of evil. Sauron is the perfect villain for Tolkien’s perfect hero because he is evil incarnate. 

The Dark Lord Sauron of Mordor, also known as the Lord of the Ring, is the trilogy’s antagonist and the titular character. Sauron created the One Ring, which he used to gain control over Middle-earth in the Second Age, but he was eventually defeated in battle by Isildor. However, he was only weakened, not killed, and when The Fellowship of the Ring opens he is searching for the Ring so that he can rule Middle-earth once more. It is important to note that Sauron is never physically present in The Fellowship of the Ring or the rest of the trilogy. Instead he is portrayed as an all-seeing eye. Tolkien does this to generate a sense of fear because all of the characters, especially Frodo, can feel him eternally watching just out of sight. 
Sauron’s only weakness is his ambition. Tolkien characterizes Sauron as someone who only thinks in terms of desire, power, and control. As a result, he is unable to understand those who seek to destroy the Ring rather than use it against him. This aspect of Sauron’s character is crucial to the narrative because it establishes Frodo as the only person who can vanquish Sauron for good.