http://arkhamcity.co.uk/viewtopic.php?f=4&t=5800 the achievement "Stop the Clock," hinted at this character...

The trophy list also hints at fucking calender man

@kingzetta: YES! I first learned about calendar man in arkham city. Best Villian ever!

Sweet. I have him on top of my tv.

Give me my fucking calander man!

EDIT: OH fuck?!?!? He is going to be in the game, oh shit...

They also showed a quick glimpse of Robin Gameplay

I think I've gotten to the point that I don't even care what villains will or will not be revealed prior to release. I just want to play the damn game now!

Calendar Man; easily the best batman villain in terms of pure off the wall weirdness- I hope he isn't just a cameo in this installment though. I'd love to see what the unreal engine does to him. Hoping Jim Gordon's estranged psycho-killer son makes an appearance too.

Calendar Woman would've been more interested...I liked the gender swap version of that character that appeared on the cartoon.

Anyways...yay more characters...lol

Looks like poison ivy is back to. A cat woman acheivement referencing the sirens with a pic of a wilted flower was among the leak that showed night wing. IGN has a image of the mad hatter pulled from the same clip.
The calander man achievement makes it sound like you could fight him on important dates, I hope this is real world dates, special challanges or missions that only come up on major holidays

Poison Ivy would be awesome... Gotham Sirens, maybe when Joker "dies" Harley, Ivy, and Catwoman will team up <3

I still hope for Hush and Clayface to make a full appearance. :(

Mad Hatter is also a character in the game

Comicvine posted some videos of the Mad Hatter and Robin here.

And let me just say, AWESOME! It also shows Robin's alternate costumes which is the Animated Robin and Tim's Red Robin costume.