Seiji Date, known as Sage of the Halo, hails from Sendai, Miyagi Prefecture, Japan. He descends from the samurai Date Masamune, reflected in his helmet design and signature hairstyle covering one eye. His family runs a kendo dojo instilling strict traditional values of honor, etiquette, and discipline, primarily enforced by his grandfather. His father, a police officer who married into the family, and his mother, second only to the grandfather in dojo authority, are present. He has two sisters: Yayoi, five years older, a medical student skilled in kendo whose teasing caused his discomfort around women; and Satsuki, two years younger, who uses informal language and lacks martial arts interest.

Fragile health in early childhood led his family to dress him as a girl, following cultural beliefs about easier female upbringing. He exhibited rebellious tendencies, disliking kendo's rigid etiquette despite enjoying the sport. To cultivate restraint, his grandfather enforced studies in avant-garde bonsai and bamboo flute. One incident involved him breaking a bonsai while playing with a sacred sword, undiscovered by his grandfather. In elementary school, peers found his pale purple eyes unsettling, prompting tears; Yayoi suggested covering one eye with his hair, a practice he maintained.

A pivotal moment occurred in junior high. After winning a regional kendo tournament, he faced a late challenge from Kazamatsuri Shingo. Upon losing, Shingo threw debris into his visible left eye. Enraged, Seiji violated etiquette by knocking off Shingo's face protector, rendering him unconscious. As punishment, his grandfather confined him to the basement, where he discovered the Armor of Halo (Korin). Shingo later confessed, sparing himself permanent disqualification but leaving Seiji profoundly affected.

Seiji wields the Halo Armor, drawing power from light sources and housing a nodachi (Korin Ken) with a shuriken-decorated hilt. Its abilities include generating intense illumination to heal injuries or blind adversaries. The armor detects concealed evil and pierces illusions, reflecting its virtue of "Rei" (courtesy/grace). His signature technique, "Rai Ko Zan" (Thunderbolt Cut/Lightning Strike), manifests as concentrated light energy for slashing or thrusting. The armor responds strongly to his emotions, with outbursts unleashing disproportionate destructive force.

Personally, Seiji speaks formal Japanese and prioritizes strategy and observation. He maintains calm composure but harbors deep-seated rage against injustice or threats to allies, particularly Ryo. His discomfort around women stems from Yayoi's teasing, though experiences with allies like Nasuti gradually soften this. Interests include kendo, F1 racing, cycling, sword maintenance, and traditional arts like bonsai.

Seiji evolves from a reserved warrior to one grappling with the armor's moral weight. Early battles reveal protective instincts; he obliterates terrain when allies like Jun or Nasuti are endangered. In the "Gaiden" arc, he endures captivity and torture in New York as Shikaisen weaponizes his armor for murder, including killing Runa's brother. This trauma leaves him feeling spiritually "soiled," fueling uncharacteristic vengeful rage upon rescue. During the "Legend of the Inferno Armor," he detects the armors' corruption and nearly attacks a corrupted Ryo before restraint prevails. By the "Message" OVA, meditating under a waterfall brings enlightenment: armor bearers sacrifice their well-being to prevent wider suffering. He relinquishes his armor’s power briefly to demonstrate sorrow cannot exist without joy, then reclaims it to accept his guardian role.