All these comments complaining about how he's stupid for allowing her to blackmail him when he's got more dirt on her than she does on him are failing to notice a very obvious fact that explains everything: He WANTS TO DO WHAT SHE TELLS HIM TO. He fucking loves it. He's a super otaku for her, and being allowed to make costumes for her and spend time with her is a dream come true for him, as he keeps repeating over and over (so it really should be obvious to every reader, seriously). I was gonna say that I kinda like the dynamic where she feels smug about having him under her thumb with the blackmail, and meanwhile he's just super happy to do whatever she wants and to protect her secret and the blackmail is actually totally unnecessary.