Taiyou

Taiyou is innocent, affectionate, and playful. Although physically frail and frequently ill, he eagerly asks Aki to spend time with him and wants to join his older brother outside despite the winter weather.

He clearly admires Aki and enjoys his company. Their snowball fight shows a relaxed side of both brothers that contrasts sharply with Aki's later life as a Devil Hunter.

Taiyou is an ordinary human child with no Devil contract, supernatural ability, or combat training. He is physically less robust than Aki and is described through the story as frequently being sick.

Taiyou first appears in a flashback to Aki's childhood. Their parents initially tell Aki not to bother his younger brother because Taiyou is not feeling well, frustrating Aki because he feels that his parents consistently prioritize Taiyou's needs.

The brothers nevertheless go outside together and have a snowball fight. When Taiyou returns to their house to fetch a glove so they can play catch, the Gun Devil's attack instantly destroys the Hayakawa home and kills Taiyou and his parents. Aki survives because he remains outside.

The loss becomes the defining trauma behind Aki's determination to become a Devil Hunter and pursue the Gun Devil. Much later, Aki's final dreamlike sequence returns to the snowy landscape of his childhood, where he imagines himself reunited with Taiyou and finally able to continue playing with him.

Aki Hayakawa is Taiyou's older brother. Although Aki sometimes resents the extra attention Taiyou receives because of his poor health, he loves him deeply and happily plays with him once they are outside together. Taiyou's loss becomes one of the central motivations shaping Aki's life.

Taiyou's unnamed parents care closely for him because of his fragile health and initially ask Aki Hayakawa to let him rest rather than taking him outside.