The assistant works for the Rug merchant at the Monkey colony. He finds his job very humiliating and was hired because the monkeys thought it'd attract customers having a trained human.
If you try speaking to the rug merchant without a Monkeyspeak amulet or a Cramulet, he will serve as intermediary, revealing that he can "speak monkey". Sometimes he might mistakenly tell the rug merchant to give you a Banana.