[SixOfSpades]

Alternatively, you can move your saved game from the Save folder to the Mpsave folder, then open it as a Multiplayer game in BG2. Once you're there, you can drop that Mage, and create a new one to take his/her place. Save your game again, move the file back into the Save folder, open up BG again and Load your Single-player games, and you're in business.

Although cav_again is right, Shadowkeeper is faster. But you can also cause a hell of a lot of damage to your Savegame if you're not familiar with it.