Irwin is Billy's nerdy friend who desperately aspires to be popular and frequently ends sentences with the word "yo". He has an unhealthy crush on Mandy and longs to gain her affection, often stalking her, but is usually turned down or threatened by her as a response to his actions. He later admits that he likes who she is due to him having a dark side much like Mandy's, but was taught otherwise by his father and eventually grew out of his bad behavior. He first developed an interest in her when Billy possessed her and told him he was neat. In one episode while traveling to "The Sands Of Time" it is reveled that he has an older brother who is mean and picks on him. In that episode though, Irwin turns his older brother into a baby when he messes with his brother's sand timer, causing his brother just disappears. In another episode it is mentioned that Irwin has a cousin named Sandy and then she appears in his pool thanks to a rhyming, talking skull that's trying to get rid of its wish-granting punishment. Irwin is part vampire, with his paternal grandfather being Dracula and his mother being a mummy. Although he should be referred to as a half-dhampir, he is instead considered to be a half-vampire. In the Halloween special "Underfist" it was revealed that Irwin could, under certain circumstances, transform into a vampire-mummy hybrid and use the powers of his new self to his benefits. Despite this, Irwin often gets picked on by Sperg and terrorized by other horrors. Adding to this is the fact that his grandmother often embarrasses him, although she really is just showing her affections for him when she does.
Harold
Harold, who is Billy's father and Gladys' husband, is an overweight man with a large, bulbous nose and a lofty black pompadour (which is in one episode discovered to house his brain). Harold resembles his son in appearance and personality, sometimes displaying even more stupidity than Billy himself. Harold's employment is unknown and varies from episode to episode (he reveals in Big Boogey Adventure that he gets fired all the time, for even the smallest of dumb acts). Despite his low IQ, Harold was accepted to Harvard and was a former Navy Seal. Harold's alter-ego is Mogar, which draws inspiration from the rock band KISS, as seen in "Battle of the Bands." Harold is also known to be afraid of Santa Claus, while Billy is afraid of clowns (coulrophobia), the Mailman and has a terrible fear of spiders (arachnophobia).
Gladys
Billy's mother Gladys is a loving, tolerant, and patient, yet mentally unstable woman brought on by having to deal with her husband, son, and the presence of the Grim Reaper as her eternal house-guest. Gladys has red hair (which is the only similar trait to Billy she has) and is usually seen wearing a floor-length, lavender dress. She considers Grim a constant threat to her son's safety. In multiple instances, Gladys is seen attacking Grim to protect her son. She has a severe facial tic which usually appears when Billy is misbehaving, when her authority is challenged, or whenever Grim is present, most likely developed as a result of her nervous breakdown. Despite her small breakdowns, ticks and ability to ignore everything when it's going very wrong, Gladys cares deeply for Billy and Harold (although when Harold gets fired for doing something so stupid or whenever Billy is really disobeying her or acting-up she does have large out-bursts of rage).She wants Billy to grow up to be a chiropractor, even though she knows that is very unlikely to happen. She is prone to small fits of insanity and obsessive-compulsive episodes. Because of all of the stresses caused by Billy, Harold, Grim and to a lesser extent Mandy, in her everyday-life Gladys is the only character in the serise to demonstrate having more control over herself when Mandy uses her own will very powerful power.
Mindy
Mindy is a gagging obnoxious girl obsessed with proving her worth to Mandy, who doesn't care for the spoiled girl (when Mindy first moved into Endsville, she wanted to become friends with Mandy but was rejected, starting her hatred for her). She considers herself superior to her classmates. Her father buys her outrageous gifts, and it's implied that her mom doesn't love her or is estranged. Mindy is red-haired and freckled and she describes herself as "much prettier" than Mandy. She frequently calls her classmates "losers." Despite her supposed hatred for Mandy, she envies her wit and adventures with her friends Grim and Billy. She's always seen in her school clothes, even when out of school.
Sperg
Sperg is the local bully, who constantly bullies Billy, Irwin, Pud'n, and other kids to no end. He is a husky boy whose preferred method of terrorizing other kids is by administering his 'lethal' wedgies. Despite his physical strength, he is frightened of Mandy because she is tougher than him. On his shoulder, he has a heart tattoo that says "Mom". It has been revealed that his mom is actually very attractive. Sperg loves Butter Cup Candies and is very sensitive inside.
General Skarr
General Skarr first appeared in the show Evil Con Carne, where he was Hector Con Carne's paramilitary commander. He is usually a cold-hearted, hateful, and harsh man, with big interests in power and world domination, yet when alone, he behaves more calmly, tending to his garden. He is missing an eye and has a scar on his face making Billy think he is a pirate. Retired from villainy (due to Evil Con Carne being bought out by a big entertainment corporation), he moves into Billy and Mandy's neighborhood and is constantly struggling with the temptation to return to his evil ways. He attempts to distract himself from his hunger for power, usually by gardening. Whilst he is no longer a member of Evil Con Carne, Skarr has many weapons and mementos from his past which he stores in his private study. Among these are his military outfit and a large picture of himself with Hector and Ghastly. In Underfist: Halloween Bash, which takes place a year later after the series ended, he has grown facial hair. General Skarr's appearance is based on that of SS Obersturmbannführer Otto Skorzeny.
Pud'n
A weak and easily frightened classmate of Billy, Mandy, and Irwin, Pud'n is often the victim and/or instigator of some trauma. He is a red-headed, freckled boy with buck teeth, and wears a hoodie. He has no parents, but was raised by wolves. Despite this, Pud'n loves dolls, bunnies, and flowers. At times, Pud'n can also be a terrifying force and show a dark side to his personality. At one point he even attacked General Skarr because Pud'n thought that Skarr killed his "bunny friends" and made Skarr run, beg and scream like a little girl. In another episode Pud'n showed a fear of the tolite and became even more scared when discovered Grim's bones in one after Billy dumped Grim in one at school and after he discovered his teacher Ms.ButterBee glued to the ceiling above the toilet in the Boy's Bathroom. He has also been known to have an allergy to pies, which cause his face to swell up. Though he was raised by wolves it is shown in one episode that he lives in a house and is a nextdoor neighbor and good friend of Billy.
Eleanor Butterbean
Ms. Eleanor Butterbean is the uncaring teacher of Billy and Mandy's school. She often takes naps in class on her desk and detests her job as a teacher. While she is mean to everyone she has been the focus of a few episodes like Substitute Teacher. In that episode Billy's ghoulish cousin Nergal Jr. is attending his first day at school and gets picked on nonstop by the school bully named Sperg. After a whole morning of tourment Jr. seeks reveange and impersonates Ms. Butterbean while the real one is glued to the boys bathroom ceiling. During which time, Jr. tourments Sperg and Billy, Mandy and Grim watch the fun. Upon which Billy notices that Sperg has the same bunny underwear that he does. Ms.Butterbean speaks in a slightly lazy tone of voice, which shows she is bored by her occupation.Once, Billy brought her a rancid apple as a gift, which nearly killed her and made Grim take over her profession while she recovered. In a few episodes, she is slightly more sensitive, although not common, in one episode Billy changes his F to an A causing the universe to turn inside out, causing her to act like this alter ego of hers.
Principal Goodvibes
Principal Goodvibes is the Paul Lynde-esqe principal of Billy and Mandy's school. His name was legally changed to Goodvibes, and he makes sure that his students feel good to the point of absurdity. It is later revealed in one episode that he sleeps in his car.
Hoss Delgado
Hoss is a spectral exterminator; a hunter of paranormal creatures. He possesses one real hand and one mechanical one, the latter of which can seemingly produce any tool he desires, most commonly a metal fist or a chainsaw-launching crossbow. He also claims to have a chip in his brain which heats up when he talks to an idiot, and when he talked to Billy he claimed that it was 'burning a hole through [his] forehead′. Hoss is the take-no-prisoner figure in Grim Adventures, though in most episodes he acts in a clumsy and/or goofy manner. For a while he dated Eris, goddess of Chaos, and their relationship was very dysfunctional. Hoss is known to make incredibly odd and usually completely irrelevant metaphors, often when talking to someone who is either cowardly or stupid. Similarly, he uses exclamatory phrases which are different, but recognizable from their original form. In his first appearances he distrusts Grim, but this dissipates as the series progresses. He resembles Kurt Russell's character Snake Plissken from Escape from New York, and is an obvious parody of Ash Williams from the Evil Dead Trilogy. His name is a parody of Jose Delgado, also known as Gangbuster is the DC Comics continuity. In Underfist, his leg was replaced with a chainsaw.
Jeff the Spider
Jeff is a giant spider who thinks Billy is his father. When Billy opens Grim's magical trunk one day, he finds an egg, which he incubates himself and raises. When it hatches, Jeff appears and believes Billy is his father. Unfailingly friendly, Jeff wants more than anything to gain Billy's love despite Billy's arachnophobia-based hate for him. Billy frequently beats him with blunt objects. Jeff blames himself for Billy's hatred toward him and constantly tries to win his love. With Mandy's help, he finally manages to scare Billy into loving him. At one point, Jeff rejects Billy when he refuses to love his "son", but it just turns out the spider was possessed by a cow-spider demon.Jeff was engaged to the Spider Queen in Wrath of the Spider Queen.
Dracula
Dracula is Grim's childhood idol and Irwin's grandfather. The once famous vampire lives in the Home of the Ancients, a retirement home for old movie monsters. Dracula talks fast and likes to dance. He does not remember names well, and often calls Grim "Skeleton Man" or even "Dummy". He is argumentative, senile, stubborn, and easily distracted and often refers to himself in the third person. When he is asked about drinking blood, he is very adamant that he does not suck, but scrapes and licks the skin instead like a vampire bat.
His relation to Irwin is that he married Tanya and was thus the father of Dick and grandfather of Irwin, a fact that neither of them were aware of. Tanya had thought he left them; ironically, he was chased out by her after assuming his bat form.