It looks like this mystery character can be unlocked with any of the new Characters from the Munchable Madness Update (Cutie Pie, Hardcore, Jabba The Hot, Sweet Beets, Lil' Softie, Tootnramen or Great Shakers). We are not perfectly sure whats the exact unlike conditions. Probably you have to kill a certain enemy like the toaster. Nevertheless it doesn't seem to be too difficult, just play a little bit.
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