"52 Shake-Up" is a series focusing on DC's New 52. Posts get a 52 Shake-Up tag if it's an already existing canon immigrant who now exists in the New 52 as well, or if it's a previously existing character who was changed so drastically by the New 52 to fit a version of themselves in another medium that they could be mistaken for being a canon immigrant.
It's always strange to see who will be a canon immigrant, and especially who will show up in the New 52. The following character was actually the first immigrant from Batman: The Animated Series (not counting Renee Montoya, who made it into comics before the show even started), even beating Harley Quinn, despite the fact that he only showed up in one episode. I wrote about him before on the Batman: The Animated Series Super Post, but I've moved his entry here:
Lock-Up first appeared in the Batman: The Animated Series episode "Lock-Up" (1994). His first comics appearance was a cameo at the end of Robin #24 (January 1996), before getting a full appearance in Detective Comics #697 (June 1996).
Likewise, Lock-Up now fully exists in the New 52, beating such characters as Cassandra Cain, Donna Troy, Elongated Man, and most of the Justice Society. He first appeared in Batman: Eternal #3 (April 2014) as part of a gang of low-rent Batman foes, including Cluemaster, Signalman, and Firefly: