That one unit in every game (or at least that I've played) who sucks terribly at first, but becomes one of the best in the entire game.
Actually, donnel is still one of the worst at the end of the game. His class options aren't particularly impressive and he has some of the worst max-stat mods in the game. That's not even accounting for the fact that you have to babysit him and he's a huge xp leech early on, providing no worthwhile returns until you've class-changed him.
Donnel isn't worth using on anything higher than normal.