Esme & Roy

• Esme (voiced by Millie Davis) aka Esmeralda is a 9-year-old girl and a monster sitter, and she has a skill for inventing activities to fix the monsters' problems, with the help of the MonsterCase, a suitcase full of fun and games that encourages the monsters to play their problems away and turns into a motorized scooter when taking Esme and Roy to where their monstersitting service is needed. Esme went through a Monster Sitter Meltdown for the first time since the episode in "Party Time", which is, so far, the only time she has gone through that, but like what she does for the little monsters she and Roy watch, Roy helps her feel better by singing the "Belly Breathing" song, which is normally sung by Esme. In "Stroller Derby", it's revealed that Esme goes to a school that's different from the one the little monsters go to.
• Roy (voiced by Patrick McKenna) is Esme's best friend and a yellow monster. He is a lovable silly monster who knows everything about monsters with his Monster Fact book. He loves to eat meatballs. Roy went through a Monster Sitter Meltdown in 3 episodes which they are: "Two Can Play That Game", "Sing Out, Roy", & "Training Day" which is, so far, the only time he has gone through that, but Esme, along with the little monsters, helped him feel better by singing the "Belly Breathing" song, which is normally sung by Esme doing it solo. But, Esme did help him feel better by singing the "Belly Breathing" song solo in 2 episodes only.
• Dumpling is Esme and Roy's pet monster. She can roll herself up into a ball and loves to dance.
• Hugo Ooga (voiced by Shayle Simons) is a theatrical 4-year-old[12] winged monster. He loves to be the star of the show and creates his own spotlight by clapping his hands.
• Fig Ooga (voiced by Elle Simons) is Hugo's 1-year-old[13] baby sister. Unlike her brother, she has no wings. While small, she has super strength and loves to pick up Roy to demonstrate it. She shrinks if she smells a Sludgeflower.
• Tillie Plink (voiced by Abigail Oliver) is an active 3-year-old[14] monster with pink fur, blue eyes, and red hair tied into pigtails with a pair of blue hairbows. She has a need for speed and loves cars and playing rough-and-tumble games and she spins a lot really fast like a dervish when she's sad, for fun, or for dancing.
• Snugs Muzzywump (voiced by Benjamin Hum) is an adorable 3-year-old[15] monster who loves to hug and snuggle with people and monsters alike. He puffs up when he gets scared, but is easily calmable. He is frightened by thunder.
• Simon Swoozle (voiced by Jacob Soley) is a 8-year-old[16] six-tentacled green monster with yellow hair, brown eyes and blue glasses. He loves enforcing routine and order. He has a big heart, is very artistic and he is very smart.

• Mr. Plink (voiced by Zachary Bennett) is Tillie's father.
• Grammy Swoozle (voiced by Jayne Eastwood) is Simon's grandmother
• Willie (voiced by Richard Binsley) is Simon and Grammy Swoozle's pet.
• Mrs. Muzzywump (voiced by Denise Oliver) is Snugs' mother.
• Frank Bleederblop & Franny Bleederblop (voiced by Justin Paul Kelly & Lily Bartlam) appear in "Two Can Play That Game". They are twin dragon-like monsters.
• Mr. Bleederblop (voiced by Jamie Waston) is the father of Frank and Franny.
• Mr. Ooga (voiced by Martin Roach) is Hugo and Fig's father.
• Mrs. Ooga (voiced by Melissa Altro) is Hugo and Fig's mother.
• Mrs. Hoozlewoo (voiced by Kylee Evans) is Sid and Lucy's mother.
• Sid Hoozlewoo & Lucy Hoozlewoo (voiced by Christian Corrao & Hattie Kragten) appear in "Imitation Frustration" and "Don't Bug Me". They can change colors, copy what they do and bounce around on their springy tails.
• Lottie (voiced by Lilly Bartlam) is introduced in "Monster's Little Helper". She wants to learn to be a Monster Sitter from Esme and Roy. She has yellow fur and flies with wings. She returns and makes a cameo in "Game Plan" to help monsters prepare for Camp Monsterdale.
• Norm (voiced by Patrick McKenna) appears in "Supermarket Match". He is a blue monster with tentacles, blue hair, and green eyes and speaks like a teenager. He appears to be a monster of all trades, as he has a different job in every episode.
• Mom and Dad: Esme's parents are, so far, only glimpsed during the opening theme of the show. They live in the house next to the garage where Esme and Roy usually hang out when they're not on their monstersitting duties and are shown waving goodbye to Esme and Roy as they set off to where they're needed. They have yet to appear in an episode and play a role for the main characters. Aside from Esme, they appear to be the only human residents of Monsterdale.

Esme & Roy has been met with mostly positive reception. Emily Ashby of Common Sense Media rated Esme & Roy a total of 5 out of 5 stars, stating that Esme & Roy's "thoughtful approach to self-awareness and mindful behavior makes it an excellent tool for families and caretakers to use in teaching similar skills to kids," and that the broader emotions will relate to a child's experiences.[19]