Charlotte is a character from the series, Madoka Magica . She is yet another witch brought into these games by, you guessed it (not), Tricky. She's just as active as Isaac, if not LESS active due to her being a secondary avatar for her submitter.
In the series she comes from, she is known as the Dessert Witch, and has a second form which is much more spookier than her doll-like 1st form. She was originally the magical girl, Bebe. She also kills off a main character in her series, Mami Tomoe.