Damn.
So from what I can surmise Mayami is probably being treated halfway between “rightful member of the main house” and “daughter of a dishonoured woman”, and that compounded with being the only heir has her being expected to abide by even stricter rules. This results in her looking down on others — perhaps reveling in their misfortune — and possibly even in her latent self-harm later turned masochism.
In her eyes, Manabu managed to “score:” he lives with gramps — her “first love” —, he's free from main house intriguing and scheming, and his life isn't even a total mess because of it! What a deal, right? Right?
What's interesting about Manabu is that, despite his apparent difficulties in expressing his deeper thoughts and insecurities, he's (involuntarily?) quite quick-witted: he even manages to make her feel the guilt of her actions and melt her mask clean off, if for a split second. One might even term him “acid”... to a fashion.