Reborn as a Flea: Suck Blood to Win

A man who has never made anything of himself wakes up one morning as a flea. To stay alive he has to bite and drink blood, and the blood is also what changes him. His mouthparts can go through anything. There is a pattern on his back, and it grows more complicated with each higher grade of blood he takes in. If it goes far enough he will grow wings and be able to fly. He is living in an academy where everyone else is far stronger than he is, and he gets by on low tricks and a will to live out of all proportion to his size. Along the way he gets on the wrong side of a young woman there. She has a cold manner and keeps a small notebook of grudges, which is already full. Her fate and his are tied together from that point, and he keeps feeding, and each kind of blood he takes is stronger than the one before.