Derrick is a student at the school.
Very little is known about him, as he's currently only present in a single scene.
According to Sarah, he smells terrible - he, supposedly, smells like a slime. He tries explaining it's because of his hair gel, but Sarah won't listen.
It's possible he'll become a datable person for Rosa in the future, though no one knows at the moment.
If Molly is your slave and you abuse it to get free items, Derrick will take over working at the item shop.