Although he first appears in Karou's sketchbooks as a creature from her imagination, it soon becomes clear that Brimstone is real. Very real. His power and influence are pretty incredible, actually—especially when you consider how little we see him or even know about him. Karou tells us that:
Brimstone dealt in wishes. Sometimes [Karou] called him the Wishmonger; other times, simply 'the grump.' (2.19)
This brief quote says a lot about Karou and Brimstone's relationship. Brimstone's basically a foster father or guardian for Karou, and their relationship is what'd you'd expect from that between a seventeen year-old girl and her dad... if the dad's expected to have dragon feet, ram horns, and crocodile eyes. Also, if he's kind of expected to be her boss.
You know, typical stuff. But Karou wants more freedom from Brimstone, she wants Brimstone to explain things to her that he won't, and (though she probably wouldn't admit it) she wants to be taken care of. That's something we can all relate to. Classic conflict: need for independence vs. need for love.
Unfortunately for our character-analyzing addiction, we don't get to know Brimstone too well. Karou says, "He'd scarcely ever shown a hint of emotion, let alone weakness or weariness" (11.12). He clearly has the patience and composure of a saint, because his job—making new bodies for fallen chimaera warriors—is not a cakewalk. The whole fate of the war pretty much rests on his scarred, human shoulders.
We do have to wonder if Brimstone's constant evasion of Karou's questions causes more harm than good. How much of the drama in the novel could have been avoided if he were honest with Karou, and just told her that she's a once-dead chimaera? We think that Brimstone might still be alive if he had just let his true feelings shine through every now and then.