Mandy is a 10-11 year old girl who typically has a cynical attitude.
Unlike Billy, she is more stable, sane, and intelligent. She'll punch or otherwise injure (physically, emotionally, mentally, or any combination of the three) anyone who gets in her way. She is Billy's "best friend" regardless of the awful way in which she treats him. She is known for rarely smiling; the one occasion on which this happens, it causes reality to fall apart. She orders Grim and Billy to do chores for her, among a slough of other grunt work. She is naturally hostile and cynical, and is shown in one episode to have physical difficulty saying the word "please."
Mandy uses and exploits Grim as her own personal supernatural slave. Typically, she abuses his powers for her own selfish purposes. While she is distant and not on what could be considered friendly terms with the Reaper, it can be inferred that they share a cold connection to one another. Often, they agree with each other when it comes to more morbid topics, and seem to somewhat enjoy each others' company. Mandy originally became Grim's friend and master through a game of limbo, wherein she proposed that if he won, he would get Billy's pet hamster Mr. Snuggles and Billy; but if she won, he would be their best friend forever.