Bloodsucker was a normal leech until Donatello, Leonardo and Michelagelo decided to use it to get back at Raphael who had been disobeying Splinter and stalled their training. Raphael was afraid of leaches so other Turtles decided to force Raphael to let the leach suck on his blood. Bloodsucker sucked some of mutagen out of Raphael and Raphael started to de-evolve back to a normal pet red-eyed slider turtle. Bloodsucker started to evolve into human-kid-of-a-thing. Later Bloodsucker sucked rest of Raphael´s mutagen. Other Turtles decided to capture Bloodsucker so they could get mutagen back to the Raphael. Finally they found Bloodsucker and started fighting it. During the battle Raphael (who looked like normal pet turtle at this point) started biting Bloodsucker thus draining stolen mutagen back. After duel between them Raphael had gained back all the mutagen from Bloodsucker and was back to his former self. Bloodsucker had also transformed back to a normal leech which Raphael freed back to the river it had come from.