Ryosuke Kaga, a second-year Momozomo Academy student living independently while his mother works overseas, initially believes his father died when he was three. He later discovers this story concealed his father's disappearance. His father is revealed as Galdarblog, a Grimwald entity who hid the missing half of the legendary sword Gram inside Ryosuke during gestation before vanishing.

His personality centers on overt lechery and frequent, accidental vocalization of erotic thoughts, earning him school nicknames like "Erosuke." Paradoxically, he maintains a chivalrous belief that all women are "treasures" deserving protection, manifesting as genuine concern alongside pervasive sexual preoccupation.

His story ignites upon encountering Lisara Restall, a Grim Reaper from Grimwald, standing in the rain. Offering her shelter leads to a spirit contract formed via the broken Gram sword. Lisara utilizes his "lecherous spirit" as energy to sustain herself in the human world, imposing a three-month life limit on Ryosuke. She promises to avert this fate if he aids her mission to locate "The Singular Man."

The contract unlocks his unique physiology. His energy regenerates rapidly through sexual arousal, particularly when fixated on Lisara's body. He develops the "Invisible Dictionary" technique—mentally flipping through dictionary pages to associate words with erotic imagery—enabling swift energy recharge, proving exceptionally potent with Lisara as his focus.

Plot developments reveal his dormant ownership of Gram's missing half. The two sword fragments fuse within him, granting full control of the legendary weapon. This power initially inflates his confidence to near-fatal levels, culminating in a failed assault against Galdarblog that leaves him traumatized. Later, during a climactic battle against Galdarblog, he accidentally destroys the device maintaining the barrier between the human world and Grimwald. This causes widespread disruption to the life-death balance, overwhelming him with guilt.

His emotional evolution shifts from generalized lust to exclusive romantic attachment to Lisara. He rejects Quele Zeria's romantic advance despite her offering her virginity, prioritizing his feelings for Lisara. After dying in combat, Lisara retrieves his soul from the afterlife by confessing her love, reviving him. They become an official couple afterward, though Lisara remains uneasy about his persistent lechery.

Post-series developments show continued attempts to initiate physical intimacy with Lisara, reflecting his unchanged nature despite their relationship. His combat role evolves into empowering Lisara through deliberate arousal during battles, leveraging the full potential of their bond and the Gram sword.