David
David is the protagonist of the novel. And as most protagonists in any twentieth-century novel, David is somewhat unreliable and a number of psychological issues. He doesn't know who he is or what he wants in life. This can be seen most obviously through his sexuality. David struggles to find what he wants in a partner. Does he like guys? Does he like girls? He has a girlfriend (Hella), but then he meets Giovanni in Paris, and everything gets, well, messy....
It is never really clear whether he really loves Giovanni or whether he hates him. It could be argued both ways, I guess, or that he does both at the same time. But what is clear is that David is too unreliable and volatile. One day he doesn't want Giovanni and then when he doesn't have him he wants him again, then he wants Hella, and then he has her and he doesn't want her anymore.... And he is like this regarding most things. There is nothing he likes completely. Sometimes he likes cleaning Giovanni's room, sometimes he doesn't; sometimes he likes Paris, sometimes he prefers America; sometimes he likes being with a man, sometimes he likes being with a woman.
He is the kind of guy that is scared of being left alone, and so he pushes everyone away; the guy that is scared of losing a home, so he never has one; the guy that is so scared of being sad, he doesn't let himself be happy. He has many contradictory and self-deluding behaviors, which make him a complex and hard-to-pin-down character.
Giovanni
Giovanni is the Italian boy that worked at Guillaume's bar (before he was fired) and David's occasional love interest. He is witty, blunt, emotional and proud. Nevertheless, remembering we only get to know Giovanni through David's eyes is important because it is hard to know how much is Giovanni and how much is David's point of view.
Here is a character that is also scared of being alone. But he handles this in a logical way (unlike David): by finding and loving a person. He loves David and wants to stay in his life. He is loving towards David and seems to be really interested in him as a person. Also, he doesn't really care what people think or where the relationship is actually going (in terms of marriage and long-term plans).
However, Giovanni is far from perfect. He is carrying a lot of grief and anger from an incident from his past (the death of his baby). This is seen throughout the novel time and time again. For example, he is very mean and verbally aggressive towards women. Also, the time in his room, when he is about to hit David with a brick. And, let's not forget, the fact that he kills someone.
Giovanni is not as volatile as David, and he knows who he is better than David knows himself, but he is still a tough nut to crack.
Hella
Definitely one of the most interesting characters—and one that provides a unique perspective in a novel mostly about men—is Hella. When she returns from Spain, there is an obvious impact on the plot of the novel. However, it is also the first time a female gets an important role in the narrative.
Hella is, similar to David, a character that has opposing and contradictory viewpoints over things. For example, in one of the first conversations she has with David, she starts saying how women and men are equal, and how women don't need men to be happy or to be defined. Nevertheless, later on, she says she'll do anything for him in order to really be a woman (by this she means to have a baby, obviously an action that needs a man).
Just because she's not as present in the novel doesn't mean she doesn't suffer from changes and inner conflicts as intense as the ones David suffers. The scene where she finds out David is gay turns out to be as heartbreaking as it is pathetic. She confesses to him that she has always known, even if she hadn't accepted it completely. Then she packs, they have one last quiet drink and leaves.
She is, in a way, one of the strongest and more "masculine" characters in the novel, which is both brilliant and ironic given the themes of homosexuality and gender roles. And, next to David, probably the most complex one.
Jacques
Jacques is a rich, old, gay man, sort of combined friend and enemy of David's. He leads a sad and lonely life, trying to get a younger guy to entertain him. A twentieth-century sugar daddy, if you please.
He is mean to David more than once, mocks him, indulges in David's suffering while they're with both Hella and Giovanni and only hangs out with David in the hopes that he will have sex with him. On the other hand, he does give both Giovanni and David money every once in a while, he helps Giovanni find David after he leaves.
Also, once again, David sort of hates Jacques because it was because of him that he met Giovanni, and there is certainly some resentment there. Thus, because of the unreliable narration, we never really know for certain the truth of Jacques' morals.
Guillaume
Guillaume is the owner of the bar where Giovanni works and where David and Jacques first meet him.
Again, being that David is unreliable and that he never really likes Guillaume, we can never know how bad he really is (if at all), but if what he thinks of him is true, well, let's just say he's not the role model human being. He is superficial, selfish, narcissist, abusive, he forces himself on younger guys, mean-spirited, and the list goes on. At one point, Giovanni confesses to David that at first Guillaume wanted to be with him, and once when he (Giovanni) was really hungry, he accepted his offer for dinner (and you know what else).
That is why neither David likes Guillaume nor Guillaume likes David.
He ends up firing Giovanni and making a show about how he stole from him (which is not true). Giovanni, proud as he is, leaves. By the end of the novel, however, it is revealed that Giovanni kills Guillaume; and being that he's from a rich family, they prosecute Giovanni, which leads to his sentence to death.
Joey
Little is known of Joey. He is an American friend of David's. They were friends while growing up. One summer night they slept together, and David had his first gay experience.
Embarrassed of what had happened, they stop being friends. Once school starts, David starts hanging out with a different crowd and starts bullying Joey.
David's Father
Another character that only appears at the beginning. David's father, after the death of the mother, turns cold and distant. He starts drinking and sleeping around. He is the kind of father that wants to be a friend more than a parent.
Nevertheless, we don't completely hate him. There's this time where David is in an accident and the father opens up to him and we can see he actually loves him. It is clear he feels resentment towards himself for not being a good-enough father.
Later on, he settles down and marries again.
When David leaves, he feels he will be closer to his Dad now that he doesn't need to see him every day. Something that doesn't turn out to be quite true, being that throughout the rest of the novel, whenever David thinks of his father, he only thinks of the money he owes him.
