She is Mandy a fanart from the cartoon. The Grim Adventures of Billy and Mandy
She is the protagonist of The Grim Adventures of Billy & Mandy. She is best friends with Billy and Grim. She is a devious girl from Endsville. In stark contrast to her cheerful, talkative, uniquely stupid best friend Billy, she is shown to be intelligent, quiet, bitter and utterly ruthless with no redeeming qualities whatsoever.