Slash was originally Spike, the beloved pet turtle of Raphael. Through his many years of life with Splinter and the turtles, Spike watched and listened, learning both ninjutsu by example and the wisdom of Master Splinter.
After Spike's accidental mutation, he and Raph eagerly upgraded their bond to a partnership, swearing to live only for each other. But Slash's new mutation was unstable, and the insane new mutant tried to kill Raph's brothers to have Raph all to himself. Raph turned on Slash for this treachery, and they once loving pair became painful new enemies.
Slash was then captured by the Kraang Hive Mind, and escaped with the help of a fellow captive, the Newtralizer. For a while, Slash formed an alliance with the Newtralizer, but Slash's insanity faded and he himself came to disapprove of his new partner's disregard for innocent life. Slash and the Newtralizer finally turned on each other, and the Newtralizer was defeated with the help of the turtles and Casey Jones. Raph and Slash's enmity came to an end, but Slash felt he needed to go his own way and couldn't return home to Raph.
In the events of The Meeting of the Mutanimals, after the Kraang Hive Mind's invasion and occupation of New York City, Slash managed to evade capture and remained an armed menace to the Kraang. He encountered Pigeon Pete, another mutant who had managed to evade capture, and they became friends. Slash formed an alliance with remaining human fugitive Jack J. Kurtzman, and launched an attack on a school where the Kraang were holding other mutants captive, specifically Leatherhead and Dr. Tyler Tockwell. Unfortunately, Slash and Pete were overpowered and are themselves captured.