Leech is a character in the Marvel Comics Universe.
Leech made his first appearance in Uncanny X-Men as a Morlock, a group of mutants whose deformities force them to live in the sewers under Manhattan. He is usually depicted as being around 12 years old (his exact age is unrevealed). He speaks in broken English and refers to himself in the third person.
Leech can dampen or completely negate for an undetermined amount of time the powers of any mutants in close proximity to him. He also seems to be able to influence powers that are non-mutant in origin. For example, during Marvel's Mutant Massacre crossover event, Leech negated the powers of the superhero team Power Pack whose abilities were bestowed on them by an extraterrestrial.
His dampening ability in the beginning was involuntary and uncontrollable, but now his power appears to be under his conscious control. In addition to leaving his victims temporarily depowered, his leeching can have physical effects. For example, at certain times when his power has been targeted at mutants with super strength, his victims have become temporarily thin and frail. Leech has also shown the ability to manifest a weakened version of a mutant power his power is directed towards.