King Seikima rules the Shikima, a demon race dwelling in the parallel Shikima Realm. His authority hinges on a magical seal compact, historically protected by the Miroku ninja clan, which preserves dimensional equilibrium despite relentless challenges from rival factions seeking his throne.

He fathered Miko Mido, the series' protagonist, with Maria—a human queen and former Miroku member. This union grants Miko demonic traits like blue blood, positioning Seikima as a linchpin in her struggle to reconcile her dual heritage. His paternal role indirectly steers her destiny when enemies assail his realm or the seal governing the Shikima.

During "Lady Blue," cursed flutes bound to vengeful spirits torture Seikima and Maria, exposing his vulnerability. This crisis forces Miko’s intervention, proving mystical artifacts can breach his dominion over the Shikima.

"La Blue Girl Returns" revolves around the Mahoroba clan’s plot to seize the Shikima Brain—a cosmic power source within his realm—by stealing the Shikima-controlling seal. As the realm’s guardian, his sovereignty becomes the conflict’s core stake, driving Miko and allies to defend it. The narrative cements his status as the Shikima’s anchor, though his direct actions remain unexplored beyond this foundational role.

Internal and external factions periodically challenge his leadership, yet the Shikima largely obey his command. While some demons harbor ambivalence toward humans, Seikima’s decrees and pacts enforce order, framing him as a pragmatic monarch rather than an innate malevolent force.