The two most obvious words which can be used to describe Squeaky's older brother Raymond in "Raymond's Run" by Toni Cade Bambara are "mentally challenged." It is true that this is his condition, but it really is not the thing that most defines him in this story.
Everywhere Squeaky goes, she takes Raymond with her; this is her chore instead of cleaning the house or doing the laundry. The thing about Squeaky is that she likes to run, really run, and she is fast. While other girls simper around and pretend not to work at being good spellers or good pianists, Squeaky does not care who sees her working on her running.
[You] can see me any time of day practicing running. I never walk if I can trot, and shame on Raymond if he can’t keep up. But of course he does, cause if he hangs back someone’s liable to walk up to him and get smart, or take his allowance from him, or ask him where he got that great big pumpkin head. People are so stupid sometimes.
Because this is what she does and because Raymond is always with her, Raymond has become a runner, though Squeaky does not realize it until she is running the important May Day race. Before she runs, Squeaky settles Raymond into a nearby swing so he can watch the race. As the race is about to begin, she sees
ole Raymond is on line on the other side of the fence, bending down with his fingers on the ground just like he knew what he was doing.
When the starting gun goes off, Raymond runs the race with her.
[O]n the other side of the fence is Raymond with his arms down to his side and the palms tucked up behind him, running in his very own style, and it’s the first time I ever saw that and I almost stop to watch my brother Raymond on his first run.
It is undoubtedly just a race for Raymond, but for Squeaky it is a revelation. Raymond is capable of learning and being coached; in fact, she considers coaching him herself. He is very fast and already knows how to breathe properly for racing, though it is just something he does that has driven the family crazy until now. He has the potential to be a very fast runner.
So, I would choose one of two sets of words for Raymond: fast runner or teachable learner.