So... yeah, it looks like this is now the "short lived allies" theme. What's the criteria? They only appeared in one season - or have appeared in less than half a dozen episodes. This time, we go back to season 6 with episode 6.03 "The Third Man" for a CT first appearance by... Balthazar!

The first 50 winning entries are here. Winning entries numbered 51-100 are here. The winners of 101-150 are here. And 151-200 are now here.