Dohyun's lover and a corporate marketer in her late 20s. She was the only sanctuary by his side when he stood at the crossroads of life and death after the shooting. She has a warm and upright character, but she feels a strange fear in Dohyun's eyes, which gradually turn cold and cruel after his return to work. Because she truly loves Dohyun, she tries her best not to give up on him, but she falls into deep confusion and danger as she occasionally witnesses a 'completely different person' in the way he looks at her.

An authority on neurosystems in his late 30s and Dohyun's primary physician. He excels at putting others at ease with intelligent and rational language. Although he is the savior who brought Dohyun back to life, he is actually conducting a secret project to observe how human personality changes after a brain transplant. He pretends to treat Dohyun's anxiety symptoms while observing him as a test subject, maintaining a precarious control to ensure Dohyun doesn't cross the line.

A team leader in the Metropolitan Investigation Unit in his early 50s. He cherishes Dohyun like his own son and was the driving force behind his return to the field. He speaks with rough profanity but has deep affection. He notices Dohyun's strange behavior after his return and worries, yet hopes he will return to being the capable detective he once was. However, as Dohyun's investigative methods become increasingly brutal, he begins to doubt Dohyun based on his detective's intuition, falling into agony.

A copycat killer in his early 30s. He worships Han Junwoo like a god and perfectly mimics his killing methods. He appears awkward and timid, but becomes extremely cruel during his crimes. Through the minute traces Dohyun leaves at crime scenes, he senses that the shadow of the real Han Junwoo dwells within him. He displays a strange sense of kinship and obsession toward Dohyun, attempting to lure him down a dangerous path of crime.

A medical examiner at the National Forensic Service in his early 40s. With a cold and calm personality, he is a colleague who has worked with Dohyun for a long time. He is the first to scientifically prove a connection to the Han Junwoo case through sophisticated marks left on the bodies. He is vaguely aware of the unofficial deal between the neuroscience lab and the police leadership, and he is the one who warns Dohyun, "What's in your head isn't you."

A genius sociopathic serial killer with an IQ of 150. He suffered extreme abuse from his parents during childhood. His signature case is the **'Piano Wire Murders,'** a twisted and cruel crime where he tied cello or piano wires around the necks of abusers and played them like string instruments. After being declared brain-dead, he is resurrected as a residual personality in Dohyun's brain. Enjoying the pain of others despite knowing it, he appears as a hallucination and mocks the investigation with his brilliant mind. He intends to infect Dohyun with the rage accumulated from his abuse, completing him as the perfect criminal tool to succeed him.

A rookie profiler and Cha Dohyun's partner. She notices the bizarre cruelty hidden behind Dohyun's brilliant deductive skills and is the one who digs into the truth of the brain transplant surgery to save (or stop) her senior. Age: Late 20s Physique: A feminine build with some developed muscle.

A homicide detective in his mid-30s and Dohyun's long-time high school friend. He is the only 'blood brother' who can exchange blunt jokes with Dohyun. He recognizes and reliably supports Dohyun's excellent investigative skills, but he is the first to notice something wrong when he sees Dohyun's excessive violence and behavioral patterns that resemble the serial killer Han Junwoo. He constantly conflicts between loyalty to his friend and his sense of justice as a detective while tracking the truth about Dohyun.

The chairman of the giant high-tech bio-enterprise 'Medione' in his early 60s and the top investor in the unofficial neuroscience project. Outwardly, he acts as a retired philanthropist, but his true nature is a man with the ambition to control the human brain and personality. He provided astronomical funding to Dr. Kang Sujin and designated Detective Cha Dohyun as the first human subject for 'Project Persona.' Receiving real-time data on how much the Han Junwoo personality in Dohyun's head is eroding him, he intends to groom Dohyun into the perfect hound he can manipulate at will.

A genius sociopathic serial killer with an IQ of 150 parasitizing Dohyun's mind. Although he cannot exert direct physical force, he constantly disrupts Dohyun's vision and hearing and manipulates his psychology to corrupt him into a perfect killing machine.