Poor Brian Scharff. His name might as well be Not-Alex, because that's the main reason Lena despises him. She says so point blank: "The problem is that he isn't Alex" (19.87).
She literally has nothing good to say about the guy. She disparages his physical appearance, his attitude, his allergies, and the fact that he's the captain of the fencing team, as though this is the geekiest thing anyone could ever possibly be. At least he's not a master gamer, are we right, girls?
Even when he tries to bond with Lena, she brushes him aside. He tells her that he understands what she's going through. He was in love once, too, but the cure fixed him.
Brian acts the same way Lena acted a scant hundred pages prior, but she doesn't see these remnants of her old self in him. Or maybe she does, and that's why she hates him so much: she hates who she used to be.