Raphael rescued Spike after he had been flushed down a toilet, and into the sewers. Spike's favorite food was lettuce leaves and was frequently seen chewing on a leaf. Raphael had a soft spot for Spike and often talked to Spike about the important parts of his life in private. This was important for Raph, as he needed Spike as a sounding board for his inner-thoughts and feelings. He could not reveal his feelings to his brothers because he doesn't want to seem "soft" to them, so he shares them with Spike, who could not speak at the time, but was an excellent listener. His brothers sometimes teased him about this. Raph often played with Spike and cooed at him, as well as allowing Spike to ride around on his shoulder, frequently using his pet for personification...
In Season 2 episode 7 Slash and Destroy, Spike mutates into Slash and tries to kill the Turtles. At first, when Raph discovers that Spike mutated from the mutagen, Slash suggests that they work together (without the others), which Raph agrees sounds good. However, he reconsiders matters when Slash starts attacking the others and collecting their bandannas. Raphael ultimately defeats Slash by pressing a pressure point on his arm. As Slash begins to fall off of a building ledge, Raph attempts to save him, but is too late and he falls off of the building. Raph then looks away. However, when he looks back, he finds that Slash is gone.
His next appearance is in Metalhead Rewired as one of the detained mutants in the Kraang prison. He is seen threatening one of the Kraang droids shortly before a mind control chip is planted on his forehead, putting him to sleep. After Metalhead frees all of the mutants, Slash proceeds to destroy a large number of the Kraang droids, including one that was about to shoot Raphael. He then nods at Raphael and makes for the portal.
Slash, both unmutated and mutated, shows a great deal of love and loyalty to Raph. Naturally, since he had been with Raph for years, he silently observed Raph and his family and listened to all of Raph's problems. Once he mutates, he becomes obsessed with removing anything he deems to be holding Raphael back, including the latter's family. Slash is even willing to harm Raph himself...to prove that he needs to let go of his more positive qualities so that he may become a "true warrior". Ultimately, while Slash is a crime fighter, he possesses no compassion or kindness and seeks to turn Raphael into a relentless killing machine like himself.