Detective Sergeant Stilwell is the supervisor of the Catalina Island sheriff's substation, a remote post often assigned to deputies who have fallen out of favor with command staff. He is a skilled investigator whose instincts sometimes clash with strict departmental orders. He lives on the island with his girlfriend and seeks to maintain order in the isolated community despite limited resources.

Kent Middleton works as a park ranger for the Catalina Island Conservancy. His position grants him extensive knowledge of the island's remote trails and protected areas, such as the local ironwood groves. He draws the attention of law enforcement when his past presence links him to multiple locations associated with missing solo hikers.

Lieutenant Lambert is a former narcotics supervisor and a decorated military veteran who served in the Marine Corps. He previously commanded Deputy Quigley on the mainland before Quigley's transfer to Catalina. He presents a polished, honorable facade within the department.

Detective Ballard heads the LAPD's Open-Unsolved Unit, working out of the Ahmanson Center. She is a pragmatic, driven investigator who values cross-jurisdictional collaboration and blends modern surveillance techniques with traditional detective work.

Captain Corum serves as Stilwell's commanding officer in the Los Angeles County Sheriff's Department. He prioritizes the department's public image and political standing over operational transparency. He frequently micromanages his subordinates and restricts their investigative autonomy.

Tash is the interim harbormaster for Avalon and a lifelong resident of Catalina Island. She possesses deep knowledge of the island's geography, ecology, and social dynamics. She provides Stilwell with a sense of stability and normalcy away from the pressures of his police work.

Deputy Quigley is a law enforcement officer recently transferred to the Catalina substation from a mainland narcotics unit. He relies on a network of confidential informants to generate leads, organizing the operation that initiates the central mystery.

Deputy Ramirez is a patrol officer assigned to the Catalina Island substation. She participates in the late-night surveillance operation that ends in chaos, suffering severe injuries in the line of duty.

Detective Simon is an investigator with the sheriff's homicide bureau and a former colleague of Stilwell. He arrives on Catalina to process the airstrip crime scene and attempts to balance his duty with the political pressures coming from his superiors.

Kalas is a solo traveler attempting to board the first morning ferry to the mainland following the airstrip incident. He becomes a person of interest when Stilwell observes his suspicious behavior and discovers a federal hold on his record.

Angela Metier is an athletic woman who vanished four years earlier while hiking on Catalina Island. The chance discovery of her belongings reopens a cold case and connects her disappearance to a string of missing solo hikers.

Laffont is a member of Detective Ballard's Open-Unsolved Unit. He assists with technical surveillance and fieldwork, bringing mainland resources to the complex missing persons investigation.

Masser is a former prosecutor who volunteers with the LAPD's Open-Unsolved Unit. He provides legal context to the team's field operations, advising them on the requirements for establishing probable cause during active surveillance.

Lionel McKey is a reporter for the Catalina Call, the local print newspaper. He maintains a deep interest in the island's unresolved mysteries and frequently presses local law enforcement for updates on active investigations.

Gwen Bassett works as a bartender at The Buffalo Nickel on Catalina Island. She shares an apartment in an isolated canyon complex with her boyfriend, connecting her to the periphery of a major police investigation.

Walter Bessemer is a former Marine who served in Iraq and currently owns an ammunition manufacturing company. He owns a private powerboat named the Bullet and maintains close ties with his former military comrades.

Molly Young is a professional hiking guide who operates in the Angeles National Forest. She adheres to strict safety protocols, including a refusal to conduct one-on-one hikes, which complicates the plans of those attempting to hire her.

Detective Trestle is an investigator with the sheriff's homicide bureau. He assists with the field work and evidence collection surrounding the airstrip incident, operating under strict departmental directives.

Harry Bosch is a retired LAPD detective who serves as an informal mentor to Renée Ballard. He passes down traditional investigative philosophies, emphasizing the importance of thoroughly reviewing physical case files from new angles.

Oliver Marquez is the owner of a local vineyard on Catalina Island. His business faces multiple challenges, including labor shortages and a controversial expansion plan that puts him at odds with the local community.

Helena Novak is a viticulturist working at the Avalon Beverage Company vineyard. She manages the difficult task of cultivating expensive French clones in a challenging environment and assists law enforcement when the property is vandalized.

Adriana is the wife of Deputy Alton Quigley. She experiences profound grief following the tragedy at the airstrip and holds the department accountable for failing to protect her husband.

Matthew is the son of the former mayor of Avalon. He harbors resentment toward the police after his father's arrest on corruption charges and expresses this anger through acts of vandalism around the island.

Douglas Allen is the former mayor of Avalon who faces imminent trial on corruption charges. His previous arrest heavily impacts his family's standing in the community and fuels his son's rebellious behavior.