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When first encountered by the Turtles, Bloodsucker was nothing more than a regular leech that has attached itself to a baby turtle. Raphael, upon seeing the leech feasting upon the turtle, tore it off and threw it away, disgusted by the creature. Unbeknownst to Raphael and his brothers, the leech had attached itself to Raphael and had begun feeding upon his blood and the mutagen within it.
Later, after trudging through a swamp, the leech fell off Raphael and swam away. While at their temporary home in Northampton, the Turtles master Splinter had a vision of a horrific monstrosity going on a rampage in the nearby town of Booder Falls.
After comforting their master, the Turtles ventured out into the swamp and saw a peculiar sight, an alligator doing battle with what looked like a giant leech.
Attempting to attack the large leech, Raphael only ended up with the creature affixing itself to his back. Before the creature could kill Raphael though, Leonardo managed to hack it apart with his swords. Unfortunately it was too late for Raphael, the leech had absorbed so much mutagen from Raphael's bloodstream that Raphael began regressing back to being a regular turtle.
Along with Raphael reverting to his original form, the leech survived and became the monster from Splinter's vision, a large, muscular creature possessing claws, a large, sucking mouth and what appeared to be multiple eyes, calling itself Bloodsucker.
While the Turtles were distracted with their predicament with Raphael, Bloodsucker went on a vampiric massacre in Booder Falls, killing dozens of people. Tracking Bloodsucker down, the Turtles engaged it in battle.
It was during this battle that the Turtles discovered that Bloodsucker was being manipulated by an eons-old man simply named Old Man River. During the same battle, the now completely un-mutated Raphael managed to climb on top of Bloodsucker where he managed to chomp into the leech and begin to drink the creature's internal fluids.
Drinking enough of Bloodsuckers innards to return to his mutated state, Raphael managed to lift and throw the now semi-mutated Bloodsucker at Old Man River, who had been regressed to an embryonic state by one of his own spells backfiring. The two beings collided and briefly merged together before disappearing into nothingness.