Haku Keiun is a central figure in The Eccentric Doctor of the Moon Flower Kingdom. He is the crown prince of the Moon Flower Kingdom, the son of the thirty-second emperor, and the late Consort De, who was poisoned when Keiun was a child. The death was officially ruled illness, but Keiun is aware of the truth and quietly seeks the real culprit. His position is precarious because his mother is gone, leaving him vulnerable to political enemies, most notably Consort Ro, who sends assassins against him and schemes to advance her own son.

Keiun is a capable and sharp-minded young man who genuinely cares about the future of his kingdom. He is tired of the kingdom's reliance on superstition and ritualistic healing, which he sees as inadequate. He is also physically disciplined, skilled in martial arts, though he lacks great stamina. During a regional inspection, his close attendant Shiei is gravely wounded by assassins in the Ran Province. In a nearby town, Keiun witnesses the young doctor Koyou perform a suture surgery to save Shiei's life. Impressed and realizing the political and practical value of such medical knowledge, he decides to bring Koyou and her assistant Shingdam to the capital. His initial motivation is to use Koyou's skills to reduce the influence of the Ring Sect, a religious group that controls healing in the kingdom and is aligned with Consort Ro.

Over time, Keiun's intentions evolve. What begins as a calculated political move gradually develops into genuine affection and respect for Koyou. He finds her straightforward, eccentric, and free-spirited personality refreshing, and he becomes protective of her. However, he struggles with his feelings and hesitates to reveal his royal identity to her, partly to keep her from being entangled in court dangers. This secrecy creates a barrier in their relationship. He also faces a rival for her attention in Shingdam, who shares a deep, familial bond with Koyou.

Keiun's role in the story is that of a patron and ally who enables Koyou to practice and spread her medicine, while also contending with political conspiracies, dangerous cults, and outbreaks of disease. He is an active participant in solving crises, often traveling into dangerous regions to investigate strange illnesses or unrest. His personal development includes learning to trust others, growing more emotionally open, and committing to both his people and the woman he loves. His notable abilities include his intelligence, political acumen, martial training, and his willingness to challenge entrenched traditions for the sake of progress.