Grand Finale
This drawing shows Mysterykid being stabbed by Yenne, Zuka will mention in his notes about mysterykid that "they appear to be injured" this event being what injured her. The way you can tell that Yenne is stabbing mysterykid is that she is represented in her drawings via a pencil.

The object in the back ground that has rainbows spurting out its side is Zukaakuz. Yenne's weapon of choice is a dagger that he not only stabs mysterykid with, but also zukaakuz which is why he is spurting everywhere. The reason Yenne stabs them is that since they are both fragments of Zuka, he believes he is preventing Zukember from being created.