Slash was once a common box turtle named Spike that Raphael rescued after someone flushed him down a toilet and into the sewers. Raph developed a soft spot for Spike and often confided private feelings to his pet, since sharing these with his Turtle brothers would make him appear "soft."
Exposed to Ooze, Spike mutates into Slash, a huge armored fighter. Although he retains loyalty and affection for Raphael, he turns out to be an unpredictable ally; Slash's mission is for Raph to purge himself of any weakness (eg, kindness and compassion) and become a "true warrior." This includes the other Turtles!
Slash also fights against the Kraang, but his ultimate relationship with Raph is still unresolved.