That's right: From June 3 to July 2, there'll be a post each day highlighting Teenage Mutant Ninja Turtles canon immigrants. Now, because TMNT canon is a little confusing, it can be hard to tell what counts as a canon immigrant and what doesn't. So for the purposes of this blog, a TMNT canon immigrant is anything (a) from the movies, cartoons, video games, toy lines, or tie-in comic series that later appeared in (b) TMNT volumes 1, 2, or 4 [because they're by the original creators] or TMNT volume 5 [because the original creators are involved, it's the new official continuity, and its goal is to use as much of the diverse TMNT lore as possible].
Angel is a former member of the Purple Dragons street gang who first appeared in the 2003 series episode "Fallen Angel" (March 2003). She lived in Casey's neighborhood, and he was able to get her out of the gang, after which she became an ally to the Turtles.
In the IDW series, she's known as Angel Bridge. This version of Angel is older and grew up with Casey instead of being watched over by him. He still rescued her from the Purple Dragons, but later she rejoined the Dragons as their leader and turned them into a vigilante group. Later still, she started wearing an exosuit and began calling herself "Nobody". She first appeared in Teenage Mutant Ninja Turtles #11 (April 2012).