Rocksteady is a mutant rhinoceros henchman in the Foot Clan, serving under Shredder alongside his partner Bebop. His primary role involves supporting Shredder and Krang's schemes, especially those focused on supernatural artifacts or enhanced mutations. During efforts to awaken the entity Dark Mu, Rocksteady and Bebop distract the Ninja Turtles and Splinter. They employ power enhancers supplied by Krang, temporarily gaining strength exceeding the Turtles' Supermutant forms. This conflict ends when environmental disasters tied to Dark Mu's emergence sweep them away.

Later, Rocksteady, Bebop, and Shredder travel via dimensional portal to Japan. Their mission shifts to acquiring the powerful MutaStones, artifacts housed within a mirror. Emerging directly in front of a bullet train carrying the Turtles, they battle atop the train. Rocksteady and Bebop initially subdue the Turtles using laser fire, but intervention by Splinter's ally Kinzō Hattori and his ninja clan forces their retreat. The pursuit of the MutaStones leads both groups to Kinzō's village, where a shrine explosion reveals a hidden fortress containing the mirror.

Reaching the fortress rooftop first, the Foot Clan secures the mirror. Ghostly intervention identifies the genuine artifact for Shredder, enabling him to harness the MutaStones' power. These stones grant Rocksteady serpent-themed armor augmentation, significantly amplifying his combat effectiveness. This empowered state allows Rocksteady to overwhelm the Ninja Turtles. However, Kinzō seizes the mirror and transfers it to the Turtles, enabling them to gain their own mutant armor. A final confrontation sees Shredder and Leonardo unleash energy blasts; Shredder initially gains the upper hand until the Foot Clan's armor augmentation reaches its time limit. The loss of power compels Rocksteady and the Foot Clan to retreat without the MutaStones.