I don't see how Brent and Guy could ever be friends. The good Guy would never forget what he did to Nicky. And the bad Guy, well, if he can't get anything out of that loser, he'll just ignore him if he stays out of his way. And he'll destroy him if he gets in the way of his interests. As for a more neutral Guy, I don't see him becoming friends with Brent either. They never got along and it would seem very forced to me if they could ever be called "friends" in any of the possible routes. Maybe they could come to a tacit peace, or even come to some kind of agreement, but I don't see anything that would point to a friendship between the two. It would be weird and hard to explain in a plausible way.
Regarding Isabella, she seems to me to be a very clear profile. A woman who had a difficult past, forced to do many things she really didn't want to do. A woman who gave up her self-respect in pursuit of a better life. That woman spent years sleeping with a person she didn't like in order to put a roof over her head and that of her daughter. That empties you as a person, it makes you bitter, and in the end it turns you into a dead person in life. In short, a person without self-respect, empty, and bitter. The perfect cocktail to turn a person into an insensitive monster for whom the end (her economic security) justifies the means (prostituting her daughter). In short, in her twisted logic, she did the same for her daughter and now it's up to her daughter to sacrifice herself for her.