Who's a character that only appeared in a handful of episodes but made a long-lasting impact on the show?

I'm re-watching Angel (Buffy spin-off) and a character, Illyria, only appears in the last 7 episodes of the show out of 102 episodes.

But she's one of the more memorable characters from the show because of her dialogue, looks, and the overall impact she had on last seasons arc. It's sometimes hard to believe she only appeared in such a small amount of episodes.

So who else would you say fits that criteria from other shows?