Mo is an 11-year-old girl who was found as a newborn floating downstream during a hurricane. Adopted by the Colonel and Miss Lana, she considers herself "three times lucky" and helps run the local café in Tupelo Landing. She is fiercely loyal and deeply determined to find her "Upstream Mother" by sending messages in bottles. Her inquisitive nature leads her to form the Desperado Detective Agency.

Dale is Mo's best friend and loyal partner in the Desperado Detective Agency. Though he occasionally makes morally ambiguous choices, he provides constant emotional support to Mo. He comes from a turbulent household but remains resilient. He grounds Mo in reality when she becomes overwhelmed.

The Colonel is Mo's guardian who crash-landed in Tupelo Landing during the same hurricane that brought Mo. He has no memory of his life before the storm and operates by a strict code of self-reliance. He openly dislikes authority figures. Despite his gruff exterior, he provides a comforting presence for his unconventional family.

Miss Lana is Mo's other guardian. She is an eccentric woman who provides a nurturing presence in the unconventional household. She deeply values their found family and lovingly crafts scrapbooks of their memories to help Mo feel connected. Her artistic nature sharply contrasts with the Colonel's pragmatism.

Detective Starr is an out-of-town law enforcement officer investigating a recent crime. His crisp suits and pressing questions make him an unwelcome figure to the insular residents of Tupelo Landing. He frequently clashes with the Colonel and attempts to keep Mo out of his ongoing investigation.

Macon is Dale and Lavender's father. He struggles with heavy drinking and exhibits aggressive behavior toward his family. His actions cause significant strain in his household, driving his older son to move out and leaving his wife to strictly manage their younger son.

Miss Rose is Dale and Lavender's mother. She works hard to maintain order in a chaotic household. She is fiercely protective of Dale, setting strict boundaries to keep him out of trouble, and she provides a comforting maternal presence for Mo during moments of crisis.

Lavender is Dale's older brother. Driven by a passion for cars, he moved out of his family's home to escape his father's aggression. He is a hardworking mechanic hoping to win races to financially support his mother.

Anna-Celeste has been Mo's rival since kindergarten. Despite their continuous bickering, she possesses a sense of fairness and occasionally bridges the gap between her and Mo when the situation calls for it.

Deputy Marla is a local law enforcement officer working alongside Detective Starr. She presents herself as sympathetic to Mo's background, using claims about her own past to build trust with the young detective while tracking the town's affairs.

Mr. Jesse is a notoriously grumpy local resident who frequents the café. His sudden and mysterious death sets the town on edge, prompting Mo and Dale to investigate the circumstances surrounding his murder.

Robert Slate is a wanted bank robber who begins lurking around Tupelo Landing. His shadowy presence raises alarms in the small town, making him a prime person of interest in the mysteries upending the community's routines.

Skeeter is an ambitious seventh-grader who acts as a self-appointed lawyer for her peers. Operating out of her mother's hair salon, she offers legal counsel to Mo and Dale in exchange for model race cars.

Miss Retzyl is Mo's teacher. She becomes socially entangled with the town's ongoing drama when she begins dating Detective Starr, bridging the gap between the school environment and the police investigation.

Sal acts as Skeeter's "accountant" in their makeshift legal business. She harbors a notable crush on Dale, which complicates her professional duties when Mo and Dale seek their services.

Thes is the local reverend's son. Observant and resourceful, he hires the newly formed Desperado Detective Agency and inadvertently provides them with crucial clues regarding the town's larger mysteries.