Tsukasa Atsumi begins as an ordinary, solitary salaryman whose life is upended by supernatural threats. His hidden lineage contains Samurai Blood, specifically identified as Ronin heritage, making him a target for demonic attacks and necessitating protection from specialized ninja clans.

Following a demon attack, he is rescued by Shizuri Ideura, a kunoichi assigned as his bodyguard. He agrees to a cohabitation arrangement, providing her housing in exchange for protection. This reveals his domestic skills, as he willingly handles household chores and cooking, accommodating Shizuri's NEET lifestyle. Initially characterized as passive, he often serves as a foil to the eccentric kunoichi surrounding him.

Their connection deepens through a forgotten past: Tsukasa and Shizuri met years prior, but his memories were erased by Saya Hazuki, while Shizuri lost hers to a separate demon attack. This shared history subconsciously influences their developing relationship.

During their cohabitation, Tsukasa cultivates mutual romantic feelings with Shizuri. A pivotal moment occurs when he encourages her to embrace her NEET identity, leading to a kiss that solidifies their relationship upgrade. Despite this shift, their domestic dynamic remains largely unchanged.

His Ronin heritage grants him the unique ability to manipulate Ley Lines, strategically enhancing the power of nearby ninja against demonic threats. While remaining largely non-combatant, his role evolves into becoming the emotional anchor for the group of kunoichi gathering around him.