Derek Smalls is a minor antagonist in the Alvin and The Chipmunks 2015 series. First appearing in the episode "Bully for You" - he is an older kid who alongside a gang of like-minded bullies took to physically abusing Simon.
Derek was the leader of the gang and unlike the others wasn't as keen on reforming after Simon tried to reason with them several times, this caused the school to rebel against Derek after he tried to beat up Simon and he was forced to relent, dropping Simon as his own gang also abandoned him.
Yet as the entire school went to party at the Chippettes' house Simon invited Derek to come along and Derek quickly changed, not able to understand why Simon was being nice to him after his actions and Simon says he does so because he wants to "be a better person" and that he didn't think Derek truly wanted to hurt him.
Touched by Simon's genuine efforts to be his friend Derek apologizes for his actions and even offers Simon a ride on his bike, the two seemingly becoming friends despite their original hostilities (But became enemies again in recent episodes and Derek goes back to his mean ways and is still a hostile bully towards Alvin, Simon, Theodore and the other students).