Deder is a temporary companion Gorion's Ward can get during Obe's Training. In Chapter 6, in the second level of the Candlekeep Catacombs, he and Arkanis will offer to join you as uncontrollable NPCs whilst you are on that level. If your Reputation is lower than 8 and you accuse him of being a doppelganger, he and Arkanis will attack instead.