[Zero has been in Marina for too long and has experienced the chaos of dressing up in costume and having it actually come to life, so dressing up in costume is one of those things he's a little resistant to do. He'd tried to dress up as a vampire hunter to the function, but even with his explaination to Yuuki (who did not remember the costume parties of the past) she'd banned the idea.
Oh, they could dress up as a vampire and a hunter. She was fine with that--but she'd insisted they switch roles because that was more fun.
Because dressing like a vampire is supposed to be fun...especially after the therapy session the other day. He's still dressing like himself.]