Born a wild rat
In his short rat life, Derek managed to save the lives of a witch and another rat, Blaine. All of that saving was out of his reputation’s character. Derek was once known as “The Destroyer” by other sewer rats due to his aggressive and vengeful behavior.
While trying to rescue Blaine from one of Stevi’s rat traps, Derek ended up caught as well. Unlike the other captured rats, Derek was not experimented on and quickly became Stevi’s scary pet rat. Derek had respect for Stevi due to being captured by her. Therefore Stevi was the only human Derek cared for. He frequently got jealous of Theo.
Fun fact: For the longest time, Derek was considered the scariest character… he no longer is though.