Actually, this is Tom Riddle cosplaying as Lulu. This is because the first sentence of the speech bubble says "Tom Morv--(cut off) commands you!" It's been a while since I read Half-Blood Prince myself, so I can't say who the bunny girl is...though, if I -had- to take a stab at it, I would say Bellatrix Lestrange just because she's obsessed. I'm not entirely sure, though.