I'm not 100% sure but it indeed seems that Chad is supposed to look dumb and act slow. After reading that part with the bird and Chad. He calls the new girl transfer girl all the time. Which is strangehes mexican, which kubo also seems to hate but a lot less than blacks
chad is usually found on the ground in a pool of his own blood
ive noticed that all these japanese manga artists are usually racist against blacks
its as if theyve never seen them and dont find nothing wrong with stereotyping them