Mr Muffin
Mr. Muffin is a victim of DID and is one ego shared between him and Valentine. Not only that but also suffers from schizophrenia, to Fight Club levels of influence, getting arrested one time from the voices in his head for breaking a microphone. His conditioned worsens when Fanboy and Chum Chum started studying in his school.
The only voice in his head that saves him from insanity is Valentine, his healer. She helps mend his mind and body at the price of controlling him every now and then, especially if there is doom or ripeful sadist opportunities are involved. Apparently Valentine was the main reason why Poopatine gave him the position of teacher.
As Mr. Muffin
He is a respectable and well mannered primary school teacher, with not much emotion to him, besides being easily annoyed or humored by Fanboy & Chum Chum to extreme degrees, being described as grouchy or playful by others around him.
As Valentine
He releases his botox filled boobs from his extremely tight corset and wears a blue wig, an eypatch and a skimpy surgeon's uniform. She enjoys being a ninja and employing operations on her patients. Subjecting them to torture so graphic that I can't be bothered to go into detail about. She also is madly in love with Morbious
Skullgirls
In Skullgirls Valentine aided with Double, Poopatine and Marie the Final Skullgirl, to bring destruction to Galaxy Hills and New Meridian at large. The reason why is simply for self-preservation and the fact he worked for Poopatine before the plan anyway.
Shortly before then Valentine's sadism helped turn Lacey into Painwheel to use as a pawn to stop anyone who defied the double and the Skullgirl.
Luckily however Valentine got her ass kicked by Peacock. She survived the whole event, and lives to terrorize another day. But it took her 4 months to manifest more commonly again, and next time, she will kill Fanboy and Chum Chum to relieve him of his burden.