- "Your powers won't work with me."
- ―Jimmy to Kitty
Leech (Jimmy) is a mutant who has the ability to nullify the powers of other mutants when they come near him.
Leech played a pivotal role as the source of a "cure" for the mutant gene. Magneto planned to kill Jimmy, who was being kept at Alcatraz Island, thus destroying the cure and preserving the mutant species. Magneto and his Brotherhood of Mutants attacked the facility, which was defended by federal troops as well as the X-Men.
Juggernaut was sent to find Jimmy while Kitty Pryde raced to save him. When Pryde discovered that she was unable to phase through walls while near Jimmy, she mocked Juggernaut into charging at them. Juggernaut tried to smash them into the wall. Instead of breaking through, Jimmy's nullifying powers removed his invulnerability and he was knocked unconscious.
- Power Negation Field - Jimmy creates a field surrounding him that nullifies the superpowers of other mutants within its range. This power, however, cannot be controlled at his will.
- Beast † - Teacher
- Juggernaut - Enemy and Attempted Killer
- Kavita Rao † - Test Doctor
- Shadowcat - Savior
- Storm † - Teacher
- Warren Worthington II - Test Doctor
- In the comics, Leech has green skin, which is an unusual feature.
- Leech is a member of the Morlocks in comics a group of mutants lead by Callisto.
- In the comics, Leech was orphaned because of his mutation, so his real name and parents are unknown.