Ken Hidaka began his career as a professional soccer goalkeeper. The early loss of his mother plunged him into depression, leading to placement in a church under Sister Amamiya's care; this Christian upbringing instilled concepts like sin and hell into his moral framework. He rose to become Japan's top J-League goalkeeper until betrayal by his closest friend, Kouichirou Kase. Kase drugged him during a critical match, causing a career-ending performance that resulted in Ken's ban from professional soccer. Later, Kase lured Ken to a warehouse under false pretenses, staged a kidnapping, and left him severely beaten inside the burning building to die. Rescued by the covert organization Kritiker, Ken was presumed dead by the public. He was recruited into Kritiker's assassin team, Weiß (operating under the cover of a flower shop), seeking revenge for his destroyed dreams.

Personality-wise, Ken displayed a cheerful, trusting, and kind-hearted nature, often coaching children in soccer. He struggled with impulsiveness and a volatile temper. Initially holding strong moral convictions, he openly expressed disdain for lies and experienced profound guilt over his assassination work—a contrast to his teammates. This internal conflict intensified, manifesting as deteriorating mental stability. Following Kase's betrayal and death (where Ken ultimately killed Kase after a repeated betrayal), Ken suffered severe trauma leading to insomnia, recurring nightmares, and growing bloodlust. By the events of the OVA *Verbrechen & Strafe*, he exhibited early signs of insanity, including violent outbursts and disregard for orders. In the *Glühen* storyline, this decline accelerated; Ken actively sought former teammates during Weiß's two-year collapse, relying on them—particularly Aya Fujimiya—to restrain his violent impulses. His aggression became more pronounced, including physical confrontations with allies over perceived slights or frustrations. Despite this, he remained perceptive regarding team dynamics, recognizing Aya's contemplation of leaving Weiß.

Ken's narrative arc extended into the *Side B* spin-off. After a period of incarceration (hinted to be facilitated by connections to the Takatori family), he reflected on his life's purpose and decided to reunite with Aya Fujimiya (known as Ran). Leveraging intelligence from Mamoru Takatori (formerly Omi), Ken retrieved Aya's katana and traveled to England, joining the covert unit Krypton Brand alongside Aya. Resuming the dual roles of florist and assassin, he investigated drug-related crimes linked to Koua Academy. His soccer background resurfaced while investigating the death of a former player friend, suspected to involve performance-enhancing drugs. Within Krypton Brand, Ken formed a protective, sibling-like bond with Kurumi while maintaining a contentious relationship with Chloe. His partnership with Aya remained central, reflecting mutual reliance forged through shared trauma.

In combat, Ken utilized modified bagh nakh—steel claws integrated into gloves—excelling in close-quarters combat. His athletic background contributed to exceptional reflexes and physical agility. Symbolically, his association with gentians (representing "you are unjust") reflected his disillusionment with betrayal and corruption.