Tenma Morimura, a modern Kyoto high school student, shares a close bond with Akane Motomiya. They, along with underclassman Shimon Nagareyama, are transported through a mysterious well to a historical Kyoto mirroring the Heian Period. In this parallel world, Tenma is recognized as one of the Eight Guardians, the Seiryū of Earth. His Thunder elemental power manifests through a dragon jewel tattoo on his left arm.

Beneath a tough exterior lies a fiercely protective nature, especially toward Akane. He initially hides the depth of his feelings for her, but his protectiveness surges when she faces threats, fueled by a deep-seated fear of losing someone precious. This anxiety stems from the unresolved disappearance of his younger sister, Ran Morimura, in their own time before the journey.

Tenma's character evolves around his relationship with Akane. Witnessing the dedication of the other guardians to protect her sparks jealousy, compelling him to confront and openly express his romantic feelings. His personal journey culminates in a direct confession of affection to Akane. He vows to remain by her side, sealing his commitment with a kiss on her forehead.

As Seiryū, he actively battles the Oni Clan alongside the other guardians—Minamoto no Yorihisa, Abe no Yasuaki, Fujiwara no Takamichi, and Tachibana no Tomomasa—using his lightning-based powers. They form a collective force defending Kyou under Akane's leadership as the Priestess of the Dragon God.