Mrs. Kerr

Mrs. Kerr is the fake identity of Robin's babysitter who is actually the Joker (Dark Age). She was beaten-up by Robin but is repeatedly rehired by Batman as a babysitter for Robin, as shown in the end of the episode Robin's Babysitter. The Mrs. Kerr costume was used by Two-Face in the episode Robin's Birthday in an attempt to get into the Batcave, along with the rest of the villians.

Trivia

 * It's obvious to find out that Mrs. Kerr is the Joker because she shares so many characteristics with him (like swearing, for example).
 * It might be possible that the Mrs. Kerr costume wig is that from Karen's hair-piece, possibly saying that the Joker killed Karen and shaved her hair and put it on to disguise herself as a babysitter.

Quotes

 * "Okay you little piece of s***, if you don't show me Batman's secret identity, I'll rip your f****** heart out, you got that?"
 * "Oh, it's nothing. I'm fine."