Timmy is a confident but highly oblivious boy who runs Total Failure, Inc., a detective agency operated out of his mother's closet. He wears a distinctive scarf and approaches the world with an intense, albeit entirely inaccurate, sense of observation. Instead of focusing on his failing grades or his family's financial struggles, Timmy obsesses over his imaginary cases and his perceived enemies.

Timmy's Mom is a hardworking single mother employed at a stationery store. Her hours have recently been cut, causing her immense financial stress which she tries to manage by riding her beloved Segway. She unconditionally loves and supports Timmy, indulging his vivid imagination by reading his self-illustrated stories to him at night.

Total is a polar bear who supposedly wandered south from the Arctic to become Timmy's closest friend and business partner. He acts as the agency's primary obstacle because he cannot write or use a phone, preferring instead to eat garbage and chicken nuggets. He sits outside the school fence during recess to wait for Timmy to finish classes.

Rollo is Timmy's best friend, though Timmy frequently insults him and dismisses his intelligence. Rollo is highly anxious and deeply concerned with his grades, dreaming of getting into Stanford one day. Despite Timmy's demanding behavior and poor treatment, Rollo remains a steadfast and logical presence in his life.

Corrina Corrina is Timmy's classmate and rival detective. She runs a well-funded, successful detective agency out of a bank owned by her father. Timmy views her as his ultimate nemesis and blames her for his misfortunes, though she barely registers his existence and focuses instead on her own work and her loneliness at home.

Molly is a classmate of Timmy's who has a massive, unreciprocated crush on him. She has mismatched pupils and frequently invents fake mysteries purely to get Timmy's attention. Timmy completely misinterprets her affections, logging her in his notebook as a potential international thief.

Crispin is the new boyfriend of Timmy's Mom. He wears a bowling shirt with an upturned collar and sports a soul patch. He also has a noticeable habit of eating raisins. He takes a strict, condescending approach to Timmy, constantly lecturing the boy to grow up and stop playing pretend games.

Mr. Crocus is Timmy's elderly school teacher. He has zero patience for Timmy's classroom disruptions and test-sabotaging, eventually warning that Timmy is on track to be held back a grade due to his poor performance.

Flo is a strict librarian whose nickname comes from the fear he instills in patrons. Despite his tough exterior and rumors that he reads dark literature, he is helpful to Timmy, assisting him in building a website for his detective agency.

Dondi is the school lunch lady and one of the few people Timmy voluntarily socializes with during the school day. She is talkative and friendly, frequently supplying Timmy with Rice Krispies squares so he can feed Total.

Gunnar is a classmate who hires Timmy to solve a theft at his home. He provides Timmy with an early mystery to solve, though the investigation stalls because Timmy ignores the obvious physical evidence in Gunnar's own house.

Gabe is Gunnar's brother and the obvious culprit in the case of the missing Halloween candy. Timmy completely ignores the chocolate covering Gabe and writes him off as having a solid alibi.

Max is a classmate who hires Timmy to figure out how his hamster died. He watches in confusion as Timmy treats the incident like a major crime scene and confiscates pet supplies as forensic evidence.