Kurt Ryder is a green-furred metamutant whose parentage and early life are thus far unknown, up until his short stint as a circus acrobat. He was soon kicked out of the circus for looking and smelling too 'freakish'.
The New Blood members of the Judgement League Avengers headed to New Jersey to investigate reports of a strange creature living in the wild. They discovered that the strange creature they were looking for was Kurt Ryder, now calling himself 'Jack'. Taking the name of Nightcreeper, he joined the New Blood of the Judgment League Avengers.
Like the rest of the New Blood members, who were actually brought together secretly by Mister X, Nightcreeper joined the JLX as a founding member, generally used as comic relief. At one point, he was caught in a love triangle with fellow teammates Angelhawk and Kokoro, which was causing problems amongst the team.
When the Armageddon Agenda arrived, Nightcreeper was among the metamutants who were imprisoned by the JLA. He also was one of the original members of the new JLX team, which was assembled by Amazon when Fin Fang Flame attacked the JLA.
It is rumored that Botanique may be Nightcreeper's biological mother.
Nightcreeper has the power to teleport short distances, leaving behind a cloud of sulfur. He can also cling to walls, camouflage himself in shadows, and his tail is prehensile.