The first thing to do is to define the word "foil" as it would apply to literature. A foil is a character whose actions or beliefs contrast with those of the protagonist. In other words, the foil is a character who interferes in some way with the "hero" getting what he or she wants. Foils are often similar to the character that they are butting up against. They usually have some qualities that are the same, only from different angles. For example, both Lex Luther and Superman are intelligent guys. He makes a good "foil" for this reason. If Luther were an idiot, he would not be Superman's match and therefore would only be a villain, not a foil.
So where does this leave us for F451? The character who most qualifies as a "foil" in the book is Captain Beatty. Physically, the two men are not the different, though Beatty is older. They both are sturdy guys who do a physical job. Both men are intelligent, even though Guy's perspective is developing. Both, as time goes on, take an avid interest in books. That's where the camaraderie stops.
It is there perspective that is different. Beatty was interested in books but "gave them up." He sees nothing wrong with burning them. In his opinion, the books only lead to confusion and disorder. Guy, on the other hand, is just beginning his interest in books. He does not see them as a pathway to disharmony, but rather is intrigued by them.
So other than this difference of opinion, what makes Beatty a foil? Well, it is Beatty who, in the end, forces Montag to burn his own house and books. It is Beatty who teases Montag with quotes from literature, and Beatty who insists that he is going to track down the other person on the end of the earpiece and get them two. In short, it is Beatty who ideologically most confronts Guy.
Why not his wife? She called in the alarm! Well, his wife is not his match. She is intellectually a midget and physically weak. Though she is the one, apparently, who calls in the alarm, she is not a powerful enough character to be a "foil."
Why not the hound? It is what actually chases after him! The hound is powerful (in many ways, more powerful than Guy) but it is just a machine. It cannot match wits with Guy and only obeys the commands of (drum-roll, please!) Beatty.
So there you have it. Guy's "Lex Luther" is Beatty.