Because Destiny Says So: Miwa starts thinking that all of her brothers will wind up becoming mutant turtles because this was the case in the various iterations of them throughout the multiverse that she had encountered.
The Millstone: Miwa accuses Casey Jones of being this, seeing him as an untrained kid who's all talk that she and her siblings have to save constantly. Later, Casey finds out that this is Anger Born of Worry and this is what convinces him to take classes at the Dojo.
