Raph was very protective of Spike; Donnie and Mikey weren’t allowed near him when Raph wasn’t there to supervise them. Raph allowed Splinter and occasionally Leo to look after the unmutated turtle, only because Raph knew that they were responsible enough not to do something stupid, or try to run some sort of creepy experiment on him.

Raph loved Spike because he reminded him of how they used to be. Normal turtles don’t go running around saving the world and fearing for their lives; they were supposed to be out in the sun basking, not trapped underground in the sewers.