Mr Smith (as the kids and the residents often called him), is Cathy's grandfather and only relative who came with her on Earth to form a new Monster Buster Club. Unlike his granddaughter, Mr. Smith rarely uses any of his extraordinary powers. He doesn't assist the MBC much but he's the first person to call whenever the MBC needs his help for making Rhapsodian antidotes and other intergalactic herbal medicines. He has an easy-going and relaxed attitude. His hobbies are gardening, taking tea and playing cards with his exotic plants.
His alien form is similar to a mollusk. His body color is dark blue - green with four limbs. To find more about him, check out his information here.
John is Chris' younger brother and one of the few people who knows about the MBC. He's in charge in using the MBC clubhouse's technology whenever Chris is busy on the mission. John really wanted to join MBC but the four, especially Chris thinks he's too young. Sometimes, he impresses the MBC by helping defeat monsters or creating his own gadgets which makes them useful or backfire to MBC.
Jeremy is Chris' acquaintance in computer club and another classmate of MBC. His interests are gadgets, video games and comics. He is in love with Cathy, while she sees him as a friend only. He often bothers the MBC in the middle of the mission just to find Cathy.
Ralph Bernie and Roy
Ralph (red - haired kid) and Roy (purple haired, plump body) are friends who make trouble together in public places like in school and park. They like skateboarding and ride their electric scooters as well. They're also Mark's friends who assist bullying Danny. They're just another MBC's classmates.
Mr. Fusster is Singletown Middle School Science teacher. He doesn't believe that aliens exist which is why Cathy argues with him. He has a white car and a skeleton model named Mr. Bones. What the MBC doesn't know, especially Cathy, that Mr. Fusster is aware that aliens do exist. He's the number one reason why Cathy gets in trouble at school.
Ms. Rollins is the Singletown Middle School principal who thinks the students are soldiers while the school is their military ground. She always add with the words like "Company halt!" and really strict with the students when it comes to rules. She's also one of the people who bothers the MBC's missions. She's a driver who owns a yellow car. It's unconfirmed whether Ms. Rollins got at least affection with any of her previous bachelors. There is only one person who gets her into soft spot - Mr. Smith.
Elton is Cathy's Rhapsodian cousin and Mr. Smith's grandson. While Cathy is good using stretching abilities, Elton's specialty is mind-reading. He's able to read minds of humans and aliens. However, he likes to blurt something to embarrass someone or to amuse himself. The MBC gets annoyed at him whenever he does that. He's also a substitute member of MBC. His MBC suit is green.
At the back of his head, Elton has a green leaf sticking on it. According to Cathy, Elton's alien form is weird looking but he retorts that it is not. His detailed information can be found here.
Mr. Bitty is a gruff person who is the store owner called "Happy Mart". He sells power tools (though Cathy says the MBC doesn't really need to buy his power tools, anyway), food, magazines and snacks. He gets angry whenever he finds out that the kids are making trouble or steal items in his shop.
Matisse is the only dog of Wendy. He resembles a weird looking chihuahua with big ears, big head, and small body. Here's a trivia: Matisse is named after Henri Matisse, a French painter who revolutionized modern art in the 20th century along with Picasso and Duchamp.
Mr. Pillsbury is the coach, P.E. teacher and temporary art teacher in Singletown Middle School. Aside from those occupation, he's also a janitor.
Mr. Bernie, or "Bargain Bernie" (as he call it himself to his customers often) is a car salesman and Ralph's father. Chris says that Ralph gets a unique sense of fashion from his dad. He only appears in two episodes. Oh yeah, his favorite car in his shop is Lamborghini XT.
A grandmother who likes to relax and wander around Singletown. In one episode, she was used as a human disguise by a purple Octovore.
They aren't appeared in all episodes, but they are part of character's hidden story.
- Byron - Cathy mentioned him in few episodes like "Beware Of Frogs" and "Sore Winner". He is Cathy's uncle. According to her, she used to help Byron to cook many Rhapsodian specialties and he often sends her Apple flies when they're in season. Sometimes, he and Cathy went to planet Gunyatto for ice cream treats every Tuesdays, Thursdays and twice on Fridays. It is suggested that Byron is a chef in Rhapsodia.
- Gleeb - Mentioned in the episode "Bubble Heads" only. According to Cathy, her uncle Gleeb is said to be the a famous investigative journalist in Rhapsodia. She idolizes him that's why she joins the school journalism club. He never let up missing a story and he usually gathers information by "grilling" (not the actual definition) until they become Tikiblinkers.
- Zirium - Mentioned in the episode "Dog Daze" only. She is Cathy's aunt. She and Cathy shares the same allergy to pets and it started in Rhapsodia-7, 250 years ago in Rhapsodian scale. It is suggested that Cathy's allergy is hereditary.
- Cousin in Lobknock - Mentioned in the episode "Frogs In Space" only. The unnamed person is Cathy's other cousin. According to Mr. Smith, that person triggered the Rhapsodian Binasal Absorbia in planet Lobknock. It still doesn't imply whether Cathy's cousin is male or female.
- Danny's Uncle - Mentioned in the episode "Secret Santa". Before Danny, Chris and Cathy go to Bargain Bernie's automobile shop, Danny reveals that he spent his vacation time with his uncle working on cars.