Raphael had four brothers but he felt as if he had five. When he found that small turtle it had been a particularly rough part in the young one's life. Yet the thought of finally having a friend was so strong he didn't care that the turtle was different than himself. He didn't care that the small ordinary turtle couldn't talk, they figured out their own way of communication. Soon the pair became more than just a mutant turtle boy and his pet ordinary turtle, they became more than friends more than best friends even, they were brothers.
Raph had grown to call his new brother Spike. "An ordinary name for an ordinary turtle." He had said when he presented the turtle to his father. He had hidden the small thing for days, sneaking him leftovers whenever he could. By the time he told Splinter it had been almost three weeks since he found Spike. Even though it had been behind his back, Splinter let Raphael keep the turtle since he had already done such a good job taking care of him.
So now Raph had someone to talk to when things got rough between himself and his brothers. It wasn't just good for then, it actually helped throughout his whole childhood and till he was fifteen. Raph talked to Spike and he new everything about Raph, he knew more than Splinter, Leo, Donnie, and Mikey combined when it came to his brother Raphael.
There were days he had wished Spike was a mutant like him. However when it actually happened there was nothing he disliked more, except maybe the shredder.
After Spike turned into Slash Raph had so many mixed emotions he couldn't even count them all. On the one hand he was glad he finally got his wish but on the other he was, and he hated himself for thinking so, upset with the turnout. He had always thought that he would just be the passive turtle he had found in one of the tunnels around his home. But that wasn't the case.
You could say it was the brain warping effects of mutagen but Raph told himself it was Spike, or Slash, doing the only thing he really knew, being aggressive. It was what he had seen Raphael doing all those years and just like Raph he did have a soft spot but it wasn't shown often, less so than Raph's. Yet it was there and Raph knew it. Deep down he was still one of his brothers.
And that's what hurt the most every time he came across Slash again. Now they weren't all that much like brothers, just friends. He almost felt hurt when he found out about the Mutanimals. His brother had his own team, his own family.
When he got Chompy he was afraid that he would take Spike's place and he would either forget or no longer care about everything the pair had been trough in the past. But as time went on it wasn't so. He treated Chompy like a baby more than his brother, perhaps like a baby brother even. He felt silly for thinking that he'd forget all the things Spike had helped him through. Even if all he did was chew on his lead and listen.
But after all, that's all Raph really needed.