Koyoi Takase, whose full name is written in kanji as 高瀬コヨイ, is a male idol affiliated with the unit WITH and attends Avocado Academy with his teammates. He possesses dark blue hair, styled longer in the back than his unit mates, with bangs parted to the left, and distinctive gold eyes. Koyoi consistently displays a calm, gentle demeanor and rarely shows anger. While maintaining a generally relaxed attitude, he reveals a subtle mischievous side, particularly evident when teasing his teammate Shogo Yumekawa.

This serene present contrasts sharply with his junior high school past, where he exhibited rebellious behavior and fought students from other schools, earning the nickname "Avocado Academy's Machine Gun." A significant shift occurred during his second year of junior high when he temporarily left both WITH and Avocado Academy's idol activities (DanPri). During this departure, he encountered Shinya Oedo and Ushimitsu, who invited him to visit YamiPri (Dark PriPara). He subsequently became the third member of the unit DARK NIGHTMARE, marking a distinct phase in his character arc before returning to WITH.

Despite Shogo Yumekawa typically occupying the center position during performances, Koyoi holds the official role of leader within WITH. His key relationships center on his WITH unit members, Shogo Yumekawa and Asahi Mitaka, and his former DARK NIGHTMARE teammates, Shinya Oedo and Ushimitsu.

Trivia notes include his birthday on December 24th, making his zodiac sign Capricorn. His favorite food is miso, especially miso soup. This preference features prominently in his solo song "SHOW ME SO SWEET RULE" from the album ALWAYS WITH YOU!!!, a track explicitly about miso soup that incorporates a pun on "misoshiru" in its title. Koyoi is classified as a cool-type idol, confirmed by his Jewel.