A 12-year-old mostly mischievous light blue cat who lives with his family in the fictional American city of Elmore. He attends Elmore Junior High in the seventh grade with his brother Darwin, who is 10 years old. Often finding himself and Darwin in various antics which end up getting them into trouble, episodes in the series' first season characterized Gumball as well-meaning in his actions, but with naive tendencies; throughout the show's progression, he has since established himself as a more serious character capable of logical thinking and prone to sarcasm.

A 10 year old orange goldfish, is Gumball's best friend and adoptive brother. Initially given to Gumball as a pet, Darwin later sprouted legs and subsequently became a full member of the Watterson family. The series' first season depicted Darwin with a tendency to be more naive and gullible than others, due to his sudden introduction into the world; subsequent seasons established his more serious side, including the ability to think logically and, in "The Words", speak his mind freely. Although Gumball can sometimes be a negative influence on him, Darwin is extremely loyal to Gumball and often helps him out of sticky situations.

An overweight pink rabbit, is Gumball, Darwin and Anais' father. Richard, a stay-at-home dad, spends most of his time sleeping, eating, and watching television. Named "the laziest person in Elmore" since 1983, he has a large appetite and is a voracious eater. Richard often serves as a third-wheel to his sons' misadventures and cares deeply for his family despite his lethargic nature. Richard was originally written for being a dog, but was finally changed by rabbit.

A pink rabbit, is Gumball and Darwin's precocious 4-year-old sister. She is considered a child prodigy, to the point that she attends Elmore Junior High with her brothers. Gumball resents the fact that she is always telling him what to do, but nonetheless loves her and acknowledges her good intentions. Although impatient with her brother, she often tags along with him on his misadventures as a voice of reason, usually having to help Gumball out of situations caused by these misadventures.

a blue cat, is Gumball, Darwin and Anais' mother, who carries the main responsibility of her family's upkeep, and is 38 years old, performing house chores and working long hours at The Rainbow Factory. A workaholic, she is often over-stressed and possesses a short temper. She acts as a guide to her sons, Gumball and Darwin when they get in a tough spot with their misadventures. Competitive by nature, Nicole is a master martial artist, and is very agile and flexible. Despite her overstressed and short-tempered personality, she is still sweet and kindhearted and shows much affection for her family.