Date Seiji, the Sage of the Halo, hails from Sendai, Miyagi Prefecture, Japan. He descends directly from the historical warlord Date Masamune, a lineage reflected in his adherence to samurai traditions and his signature hairstyle covering one eye. Primarily raised by his strict grandfather, he underwent rigorous training in kendo, bonsai cultivation, and shakuhachi flute playing, disciplines intended to instill discipline and self-control. This upbringing emphasized traditional Japanese etiquette, resulting in a formal speaking style and a profound aversion to dishonesty and injustice. His mother worked as an emergency room doctor and his father served as a police officer. He has two sisters: Yayoi, whose persistent teasing fostered his discomfort around women, and Satsuki, who possessed a more relaxed demeanor. A fragile infancy led his family to dress him as a girl—a practice he has no memory of—based on a superstition believed to ease a child's upbringing.

Personality-wise, Seiji presents as calm, observant, and strategically minded, carefully analyzing situations before taking action. He places high value on loyalty and responsibility, particularly toward his fellow warriors, though he initially resisted including non-combatants like Mia and Yuli in the group due to safety concerns. Despite his composed exterior, witnessing evil or harm to allies can trigger intense rage, unleashing attacks of disproportionate power. His personal interests include avant-garde bonsai, F1 racing, cycling, and sword maintenance. Interactions with women consistently provoke noticeable anxiety, a trait directly stemming from his sister Yayoi's influence.

He discovered the Halo Armor (Korin) while enduring a kendo-related punishment in his family's basement. This armor embodies the element of Light and the virtue of Rei (Courtesy, sometimes adapted as Grace or Wisdom). Its primary weapon is a nodachi (Korin Ken) featuring a shuriken integrated into the hilt. His signature technique, "Rai Ko Zan" (Thunderbolt Cut or Lightning Strike), channels light energy into a concentrated slash or thrust capable of both destruction and restoration, such as healing Ryo's blinded eyes. The armor passively detects hidden evil and dispels illusions, acting as a beacon against darkness.

Throughout the conflict against the Evil Dynasty, Seiji demonstrated tactical acumen and protective instincts, especially toward Ryo. Key actions included freeing Kento from petrification at the Throne of the Gods, identifying Dais' illusions to prevent infighting among the warriors, and ultimately sacrificing himself to Talpa alongside his comrades. In the Gaiden OVA, he was captured in New York by Shikaisen and a mad scientist. Enduring torture, he was forced to watch as his corrupted armor committed murders, including that of Runa's brother. This ordeal left deep psychological scars, manifesting as hyper-sensitivity to the armor's potential for corruption and a temporary descent into vengeful fury upon his rescue.

The Legend of the Inferno Armor OVA highlighted his growing unease with the armors' inherent dangers. He sensed Mukala's corruption early and nearly unleashed his rage before recognizing the darkness within his own equipment, compelling him to stand down. His character arc culminated in the Message OVA. After meditating beneath a waterfall, he concluded the armors perpetuated suffering and initially rejected their power. Confronting the spirit Suzunagi, he argued that sorrow and joy are inseparable, demonstrating this by willingly reclaiming the Halo Armor's energy to don a new armor—symbolizing his acceptance of the burden required to protect others' peace.

His significant relationships include deep loyalty to Ryo, often assuming a protective role, and a close intellectual kinship with Rowen. His dynamic with Anubis, the Warlord of Darkness, remained one of elemental rivalry rather than outright enmity. Family interactions, particularly with his sisters, profoundly shaped his reserved nature and complexities regarding gender dynamics.