Blue Tengu, known as Tatsu, is the priest of a local shrine in a village facing submersion from the Hotarugaoka Dam project. Villagers and the protagonist Yuuta recognize him for his wisdom and spiritual guidance. During Yuuta's unexpected time travel to 1977—one month before the village's abandonment—Blue Tengu provides stability and insight.

As spiritual leader, he oversees cultural traditions like the final summer festival, sustaining communal morale and practices amid their home's impending loss. He emphasizes acceptance over resistance to the dam, positioning himself as a voice of resilience and continuity for the transitioning community.

Blue Tengu's interactions with Yuuta help the boy navigate the village's emotional and cultural landscape, aiding his integration. Yuuta forms bonds with Saeko (regarded as his cousin), Kenzo (a close friend), and a grandmother figure, facilitated by Blue Tengu's counsel. This guidance helps Yuuta grasp the value of transient experiences and preserving memories, particularly of the "ocean of fireflies"—a natural phenomenon cherished by Yuuta's late father.

The character embodies themes of letting go and honoring impermanence, aligning with the narrative's focus on loss and legacy. His guidance frames Yuuta's journey not as a mission to alter history but as an opportunity to forge enduring, timeless connections. Blue Tengu's acceptance of the village's fate reinforces the story's emphasis on finding meaning in fleeting moments.