Kaiō Retsu, originally named Eishū Retsu and also known as Shaoron Retsu during his training, was born in China. He trained at the Bailin Temple in Heilongjiang, specializing in the 4000-year-old martial art of Chinese Kenpo and being groomed as its heir. His master, Kaiō Ryū, initially denied him the elite title of Kaiō, criticizing Retsu's skills as insufficient compared to the legendary feat of Kaiō Dorian carving a tunnel barehanded overnight. Retsu later earned the title, possibly through Dorian's recommendation. He debuted as an antagonist in the Underground Arena's Maximum Tournament, confronting karate practitioner Katsumi Orochi and demanding he admit karate's inferiority to Chinese Kenpo. Retsu defeated Sergei Taktarov with the Spinning Lotus technique, overwhelmed wrestler Mount Toba by targeting his knee, and brutally overpowered Katsumi using the blinding "Sunkei" strike. He advanced to the semi-finals but lost to Baki Hanma after a neck-breaking counterattack. This defeat marked a turning point, shifting Retsu from arrogance towards becoming a more open-minded ally. His evolution continued through later sagas. He aided Baki by bringing him to China to cure poison and compete in the Raitai Tournament. Retsu later fought the prehistoric warrior Pickle, losing the battle and his right leg, which Pickle ate as a "gift" for his performance. The injury resulted in a pegleg that did not impair his mobility or combat effectiveness. He subsequently trained Katsumi Orochi to prepare for his own fight against Pickle, embracing a mentor role. Retsu's philosophy shifted significantly. Initially dismissive of other styles, he developed respect for diverse disciplines after befriending Katsumi. He taught Chinese Kenpo at the Shinshinkai Dojo while learning from other martial arts. Later, he entered a boxing gym to promote Chinese martial arts globally. After accidentally destroying a punching bag during a strength test, he participated in exhibition boxing matches, demonstrating damage absorption, full-power strikes, and lightning speed. He earned substantial sums but remained indifferent to the money, fostering an appreciation for Western fighters. Physically, Retsu was a muscular man approximately 176 cm (5'9") tall and 106 kg (233 lbs). He had black hair typically tied in a single braided pigtail, thick scythe-like eyebrows, and a tanned complexion. He often fought shirtless, wearing dark blue or black monk pants and boots. After losing his leg to Pickle, he used a pegleg without functional compromise. Intellectually, Retsu possessed keen analytical abilities, swiftly deciphering opponents' techniques and weaknesses during combat. He studied martial arts history and integrated insights from other disciplines into his style, ranking among the series' smartest fighters tactically. Retsu's personality blended discipline with impulsiveness. He upheld Chinese Kenpo's honor with intense pride but displayed combat impatience and occasional absurdity, like throwing a frustrated windmill punch. Despite his composure, overconfidence and eagerness to challenge powerful foes contributed to his death; Musashi Miyamoto overcame his defensive Xiaoli technique, severing his spine and organs. Retsu also showed profound compassion, pitying the mentally disabled Kaiō Dorian and promising him candy while holding back tears. In his final arcs, Retsu sought further growth through global martial arts exposure while honoring his roots, cementing his legacy as a warrior who evolved from rigid traditionalism to enlightened versatility.