Derrick is Ducky's twin brother. He is less adventurous than her and also not as sweet. He is jealous of Spike and the attention that Spike gets from Ducky. He believes that Spike should stick up for himself and tells Ducky off when Ducky keeps sticking up for him. He has even insulted Spike before.
His relationship with his twin sister is very strained. They have even come to physical blows before. Ducky said she'd prefer a Fast Biter for a brother over him and has strong dislike for him.