Billy (Billy and Mandy)

William "Billy"
First appearance	"Meet the Reaper"
Last appearance	"Underfist: Halloween Bash"
Information
Gender	Male
Occupation	Student at Endsville Elementary
Relatives	Gladys (mother), Harold (father), Aunt Sis (paternal aunt), Nergal (paternal uncle via marriage), Nergal Jr. (first cousin), Jeff the spider ("son"), Del Uglio (Perhaps long lost twin brother).
Nationality	American

Amanda "Mandy"
First appearance	"Meet the Reaper"
Last appearance	"Underfist: Halloween Bash"
Information
Gender	Female
Occupation	Student at Endsville Elementary
Relatives	Phil (father), Claire (mother)
Nationality	American

The Grim Reaper
First appearance	"Meet the Reaper"
Last appearance	"Underfist: Halloween Bash"
Information
Nickname(s)	Grim, Grimey, Grimuel, Mr. Grim, Mr. Reaper, Bonehead, Loser, Bag of Bones, G-Man, Grimmy (when younger)
Species	Skeleton
Gender	Male
Occupation	The Reaper of Souls
Title	Master of Life and Death
Relatives	Miriam (mother), Earl (father), Kali (maternal great aunt), Granny Grim (maternal grandmother)
Nationality	Jamaican

Irwin
First appearance	"Get Out of My Head!"
Last appearance	"Underfist: Halloween Bash"
Information
Gender	Male
Occupation	Student at Endsville Elementary
Relatives	Dick (father), Judy (mother), Dracula (paternal grandfather), Tanya (paternal grandmother), Melvin (elder brother), PoppinLockin (Maternal Grandfather)

Gladys
Information
Gender	Female
Occupation	Housewife
Nationality	American

Milkshakes
Information
Gender	Male as Cat
Occupation	N/A
Relatives	Billy (owner)
Nationality	American

The American animated television series The Grim Adventures of Billy & Mandy features a cast of fictional characters created by Maxwell Atoms. The series, which originally aired on Cartoon Network from June 13, 2003 to October 12, 2008, follows two children, a dimwitted boy named Billy (voiced by Richard Steven Horvitz) and a remorseless girl named Mandy (voiced by Grey DeLisle). After winning a limbo game to save Billy's pet hamster, the two gain the Grim Reaper (voiced by Greg Eagles) as their best friend for eternity, and come across a variety of supernatural or otherworldly characters and locations throughout the series.

Voiced By Richard Steven Horvitz

William "Billy" is a slow-witted, happy-go-lucky child. He has an extremely low IQ of -5, having been outperformed by a shovel and two candy bracelets on an IQ test. He has an extremely large, pink nose inherited from his father, Harold, wears a blue and white-striped shirt and covers his ginger orange hair (which is inherited from his mother, Gladys) with a red cap. His main affiliations are Grim and Mandy. Mandy is Billy's best friend, though she treats him more like a servant than a friend. It has been hinted several times that Billy may have underlying feelings for Mandy that even he (in his stupidity) may not fully realize. Billy is far kinder to Grim than Mandy is, and while he almost always goes against the Reaper's advice and uses him as a plaything, he appears to genuinely like him and often tells Grim that he's his "best friend".

Due to Billy's foolish desires, which he orders Grim to bring about, he is often the cause of Grim and Mandy's mishaps. Although generally good-natured, he has occasional fits of stubbornness, violence, and rage, which shows insanity that even Mandy seems to fear. It is suggested that while Billy is the most innocent character of the three, he ends up causing the most problems due to his irrational and often impulsive choice of actions. Billy has a wide curiosity which gets him into situations he often can't get himself out of without Grim and Mandy coming to his rescue. Billy is terrified of clowns to the point of having recurring nightmares about them, although eventually, due to an encounter with his inner frat boy, his fear is transformed into a psychotic loathing and hate for them. He is also scared of spiders, which is quite unfortunate because a huge Spider named Jeff believes that Billy is his "Father", and of the mailman. His greatest fears were combined by Horror's Hand into a monstrous clown-spider-mailman hybrid in the Big Boogey Adventure movie.

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Wikipedia