hmm...those are all very complicated relationships. Although Jake considers all of them as friends, you can sense the animosity that he has towards Brett and Robert. This probably has more to do with Jake himself than with the others.
Starting with Brett, Jake and Brett have an odd relationship. Although Brett refuses Jakes advances, it doesn't seem as though he minds that much. It may be that they both know that a romantic affair is impossible given Jake’s disability and Brett’s pattern of destroying the men in her life. Clearly there is a past between them, and a clear affinity for each other. However, in describing Brett to Robert, he calls her a drunk. Their relationship is loving, but with a slight taint for Jake because of his impotency.
Jake's relationship with Robert is more clearly defined because Jake himself talks about it in the beginning of the novel. ALthough they are friends, Jake doesn't really seem to like him. He considers him arrogant, needy, unattractive and inept with woman. As the novel progresses Jake looses sight of whatever reason he was friends with him in the first place and dislikes him to an extreme.
Jake's relationship with Bill is probably the most pure, Bill is a good friend to Jake, he tries to cheer him up, and he takes nothing from him. Bill is one of the few positive characters in the novel. He is a good person in Jake's life.