You were adopted by Splinter (you can be a mutant or a human)

Around the same age as Raph, you got along with Leo, Raph, Donnie, and Mikey.

You were in the dojo, watching the boys go against each other.

It was Donnie vs Leo and Raph vs Mikey.

Eventually, Raph and Donnie were out, and it was only Mikey and Leo standing