Senki functions as a dedicated protector within the narrative, expressly engineered to safeguard the central figure Jiro Shutendo. His debut occurs during a spacetime disruption where he transports an infant Jiro while battling a rival oni entity. Following this confrontation, Senki delivers the child to human couple Ryuichiro and Kyoko Shiba, vowing to reclaim him after fifteen years—establishing the core timeline for unfolding events.

Physically, Senki manifests as a massive blue-skinned oni crowned by formidable horns, adhering to traditional oni depictions within a humanoid framework. His combat prowess erupts through brute-force strikes capable of devastating environments. Diverging from typical bestial oni portrayals, his design implies strategic intelligence alongside raw power.

The character's singular drive centers on ensuring Jiro's survival to maturity. Narrative context confirms Senki was created exclusively for this protective role, operating alongside fellow guardian oni Goki. Their mission involves shielding Jiro from annihilation by hostile forces, particularly the demon-worshipping cult Ankoku Jashin Kyo and its leader Yonen Majari.

During the fifteen-year interim, Senki maintains an absent yet implied presence. The narrative omits active monitoring of Jiro's adolescence, concentrating instead on his return coinciding with escalating supernatural threats against Jiro and his adoptive family. This reappearance reignites conflict with adversaries while compelling Jiro to confront his non-human heritage and destiny.

Across adaptations, Senki's development remains consistent—preserving his protective function without personal backstory expansion or significant evolution. His actions primarily propel external conflicts rather than internal growth, serving as both catalyst for Jiro's journey and embodiment of ancient forces intersecting the human world.