Kaito Yaguchi attends Umezono Academy in Tokyo as a high school student. He has tan skin, caramel-brown hair cut shorter on the sides and back where it appears darker, and dark-brown eyes. His typical school uniform includes a white dress shirt paired with a blue tie adorned with slanted sky blue lines.

Kaito actively participates in the school's Tsugaru Shamisen Appreciation Club. His involvement centers on playing and engaging with the shamisen alongside fellow club members.

The name "Kaito" combines the characters for "sea" or "ocean" (海) and "person" (人). His surname "Yaguchi" derives from "arrow" (矢) and "mouth" or "entrance" (口). His parents are adult figures, with his father working as a lawyer.