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].
Null first appeared in Teenage Mutant Ninja Turtles Adventures #15 (September 1990) as an evil businessman. Over time, readers learned that he was a demon - possibly the Devil himself - and each act of evil he did caused him to become more demonic-looking. He became the primary antagonist of the Mutanimals and one of the deadliest Ninja Turtles villains around.
Null is also the antagonist of the Mutanimals in the IDW series, first appearing in Teenage Mutant Ninja Turtles: Mutanimals #1 (February 2015). She's female this time, but beyond that she's still a demonic, evil businessperson.