The boys react to rumors that their kindhearted lover comes from a long line of dark wizards/witches and they confronted her to find out it's true. That could explain her interest/proficiency in the dark arts from the dark magic book. Please?
Muns!: Interesting ask ;) Enjoy ^^
Shu: Honestly, he wouldn’t give a shit. But if anything he’d be pretty impressed, and try to challenge her with spontaneous attacks throughout the day.
Reiji: Would conclude that she has nothing to be ashamed of, because power is power no matter where it comes from. But he’d certainly use it as an excuse to lecture her on the poor quality of her spells (which actually aren’t that low quality) and insist to bring her up to both his and her family’s standards
Kanato: Would just fall in love with her even more, and urge her to use the skills her family is so infamous for so that they could run off and rule the wizarding world together
Ayato: Would think it was scary not that he’d ever tell her that but most of all he’d think it was pretty cool, and would want her to teach him some top level shit. (She’d refuse, of course)
Laito: Laito would tease her about how absolutely terrifying she is, and how it sends shivers down his spine just thinking about it. He’d purposely poke fun at her to see her snap.
Subaru: Wouldn’t think much of it, and assure her that he understands how family ties don’t necessarily define a person.
Ruki: Would implore her to use her magic wisely, telling her that reputation is everything, especially in regards to kin.
Kou: Would be rather intrigued, and probably put her through a bit of punishment for not telling him about her past. “That’s the same as lying, isn’t it, M-Neko-chan?”
Yuma: Would give him the heebie jeebies. He’d tell her to stay away from that ‘dark shit’ because nothing ever comes out of it. It’s like trying to take a shortcut and he looks down upon people who don’t work hard to achieve the fruit of their efforts
Azusa: Would be understanding and empathetic to a troubled past and would naturally be protective of his s/o and any sort of prejudices that might come their way.
Carla: Would make sure to remind her to keep her interest in check, as there is a fine line between curiosity and stupidity. He’d insist her familial background has no say in the matter, that she is good at the dark arts because she has mastered light magic as well and it is therefore only natural.
Shin: Would tell her to just forget about all that bullshit because no one can be stronger than he is anyway. But if she ever tried out any dark magic on him, she’d be in for some divine punishment…