Harold is Billy's father. Harold is where Billy gets most of his traits from, most noticeably his giant pink nose and idiocy.
He apparently has no job, because in one episode he is a doctor, while in others, he is said to have worked at Freckle Burger but got fired for eating all the paper cups so his employment is currently unknown, though it was mentioned that he was once a Navy SEAL and possesses traits that could back up this claim (can fish in a general sense and possesses an understanding of dangerous or traumatic situations with a calm, commanding ease). He was also in a rock band once, and changed his name to "Mogar". He is one of the people featured in the slideshow of who stole Grim's scythe in Billy and Mandy's Big Boogey Adventure.
Like Billy, Harold is very stupid and is prone to do stupid things. It seems he is an expert in dancing in the episode Billy Idiot, he beat the Mrs. Pollywinkle, who was Billy's dance teacher. He sees Billy as a genius due to the boys' "genius" when Billy was young and tries to get others to see this too, as seen in The Problem with Billy, much to Billy's irritation. Despite this, he still gets annoyed sometimes by Billy's antics. He is afraid of Santa Claus and his wife, Gladys, who is mentally unbalanced.
Harold is a voracious eater, and willing to eat anything. He once said that he was still so hungry after eating two breakfasts that he couldn't wait til lunch and so knowingly ate his wife's mucus covered pet rat live. He also ate his wife's goldfish as well but with a different reason that only his wife heard and not Billy, Mandy, Grim or the audience.
Harold has the same facial shape and large pink nose as his son. He wears blue pants and an a white t-shirt with orange sleeves. He is also very fat, thus, his stomach bulges out of his shirt. He is also show in some episodes to be very hairy and disgusting. Harold has 2 and a half feet, as he has half a foot growing out of his right foot.
While Harold is an idiot, he is still noticeably smarter than Billy in some ways. In The Taking Tree he received a Harvard acceptance letter which was taken by the Taking Tree in his yard before he got to it. The Harvard acceptance letter makes it entirely possible that Harold is academically a genius, but lacks any sensibilities, whereas his son lacks both of these.
He passed along the ability to dance ballet excellently to his son. His pompadour actually contains his brain, and in earliest episodes he had appeared more intellectual, but as the series went on he appeared to get dumber.
He has his own lucky pants. He said he wore these the day he asked Gladys to marry him. When Grim asked, "You call that lucky?," Harold stated that they are lucky because they're impervious to lasers, supernatural powers and mustard.
- Harold and Sis's father appeared during a flashback in the episode "Billy Idiot". He was angered by his son's passion for dancing, calling him a "Big fat sissy". He has the same facial shape as his son and grandson including the large pink nose.
- For the first few seasons, he referred to Grim as "Grimy", before suddenly getting his name right about halfway through for no apparent reason.
- Harold is voiced by Richard Horvitz, who also plays the voice of Billy and various others.