Here is a character that just might be added to either the pilot or the series itself. This character, named Damien the Murderous. (Sorry for messing up on the writing on the drawing.) a personified representation of Arthur's inner anger, hatred and malice.

Though thinking about it now, I might use Damian as the short's villain. Seeing as how it might start where Arthur would be in a more cynical and snarky state. But on the inside, he would be in a constant battle between him and Damien. However with the help of Charlotte, he would overcome this demon and finally have some peace of mind and let go of his past mistakes.