Kōichi Mizuki is a Japanese high school student celebrated for athletic skill, academic excellence, and popularity. He captained his school's basketball team for three years, securing victory at the National High School Basketball Competition while maintaining top grades. Physically, he has tan skin, brown eyes, and short black hair styled outward at the top, reaching his ears. His typical attire includes a school uniform of a black jacket with yellow buttons over a white undershirt and brown high-heeled shoes. Orphaned young, he lost his father at age three. His mother later died from overwork during a business trip, leaving him in the care of an uncle he sought not to burden. Before vanishing from Earth, he held romantic feelings for classmate Anzu Tanaka and intended to ask her on a date. His life shifted when summoned to Eustil's titan kingdom of Tildant by Prince Caius Lao Bistail during a private moment in his bedroom—contradicting earlier accounts. Caius proclaimed him his predestined bride under a prophecy forbidding marriage within their world. Resisting due to physical disparity and the non-consensual summoning, Kōichi negotiated a one-month trial. Caius vowed no coercion and promised Earth return if Kōichi rejected marriage afterward. Kind yet stubborn about unilateral decisions, Kōichi traveled with Caius during the trial, growing to comprehend titan culture and foster genuine affection for the prince. He demonstrated empathy by stopping Caius from killing Palo, a beastman who abducted him, insisting no one be harmed for his sake. He also reconciled with Caius's ex-fiancée Medina after misunderstandings about causing her distress. Facing Earth return, he discovered the dimensional rift from his summoning would erase his existence to stabilize realities. After emotional conflict over past ties, he chose to stay with Caius permanently, forging a binding contract. This manifested a heart-shaped mark below his stomach, marking him as Caius's true bride and granting access to the prince's magic. Royal life brought challenges: insecurities over meeting Caius's needs and political tensions from his inability to produce heirs. He befriended Palo and Baro Barows, bonding through basketball and studies. His persistence emerged when he joined Caius in Fobal, a beastman country, to find a cure for his illness despite risks. In spin-off combat scenarios, he employs basketball-inspired techniques enhanced by Caius's magic. These include precision ball attacks, mana-infused projectiles inflicting debuffs, and Titan's Blessing—a damage-reflecting shield. The pact boosts his agility and damage output, incorporating tactical concentration management. His journey centers on evolving from reluctance to wholehearted commitment, embracing his role while tackling cultural barriers and personal doubts. He remains resolved to forge a future with Caius despite societal pressures and the irreversible loss of his original world.