Takumi Hayami is a 16-year-old male with brown hair, grey eyes, and a height of 168 cm. He serves as a communications officer aboard the naval destroyer Koryu, monitoring Sonic Diver units and potential enemy activity. Alongside this military role, he works as an assistant in the ship's galley under Chef Gen-san, performing kitchen duties with the goal of eventually taking over his family's restaurant; he views his military service as a temporary step.

His personality is consistently friendly, gentle, and characterized by a perpetually smiling demeanor. He tackles all tasks enthusiastically, spreading positive morale among the crew. His open nature makes him one of the few males with whom Karen Sonomiya interacts comfortably, significantly helping her overcome social anxiety around men.

Photography is his primary hobby. He frequently carries a camera, capturing images of buildings, landscapes, and friends, a practice he continues throughout his service on the Koryu.

He develops a mild romantic interest in Karen Sonomiya, noted across multiple sources. This dynamic contributes to Karen's gradual increase in confidence when interacting with males.

While no major developmental arcs are detailed, his supportive roles in communications and the galley remain consistent. He contributes to interpersonal dynamics within the narrative, particularly through his positive influence on Karen and his embodiment of normalcy amid military operations.