Yes, Edwin is an Evil mage, but he is not the one outside near the Harper's hold. That is Xzar, who wants you to find his buddy Montaron, inside the Harper's hold.

Edwin is in Mae'Var's Guild hall. he has some quests for you (as part of proving Mae Var's guilt against Renal Bloodscalp). Once you do Edwin's quests, he will offer to join your party, and he is the best mage NPC in the game.