In the original Teenage Mutant Ninja Turtles comics, Peter Laird created the characters with Kevin Eastman.
Eastman and Laird developed the concept in Northampton, Massachusetts, during the early 1980s. Their first issue was deliberately self-published through their company, Mirage Studios, and appeared in 1984. The comic began as a black-and-white parody and homage to popular comics of the period, but its characters soon developed their own mythology.
Peter Laird and Kevin Eastman are often described jointly as the creators of the franchise. Laird’s name is the requested answer because the question asks for the partner who created the turtles with Eastman. Their original comic was much darker and more violent in tone than many later cartoons, films, toys, and children’s adaptations.