Actually a pretty deep name for Yuu. She has a harem within the school where everyone is wearing a uniform. She's also wearing her school uniform to a vampire meeting. The Japanese seem to believe it's a student's duty to use the school uniform also outside of the school in formal events that haven't got anything to do with the school or studying, but why would ageless vampires care about such a thing? She brought it upon herself, 100%. Just like Rin. As a matter of fact, I reckon Rin wouldn't even mind being called Tracksuit Vampire if the other vampires didn't use the name with the intention to mock him. He's the chief missionary of the tracksuit religion, after all.