Yaku is a kunoichi of the Miroku clan, encountered by protagonist Miko Mido upon her arrival at the clan's hidden village. She survives alongside Miko when renegade ninja Kugutsumen and the vengeful former Miroku instructor Kamiri massacre the rest of the clan. Yaku features notably large G-cup breasts, which provoke Miko's envy, green shoulder-length hair, red eyes, and sometimes wears a hair ribbon. Her apparent age is around sixteen. Despite this, she exhibits a mature personality and high sexual activity, preferring relationships with "yankee"-style delinquent young men who often fail to satisfy her intense carnal urges.

Yaku possesses a supernatural condition: on full moon nights, she undergoes an involuntary transformation into a werewolf unless she achieves orgasm beforehand. This lycanthropy renders her both a powerful ally and a potential threat. As a trained Miroku kunoichi, she is skilled in "sexcraft ninjutsu," a combat discipline utilizing sexual techniques. This training included casual lesbian encounters with fellow clan members to hone their abilities. In battle, she and other Miroku kunoichi often fight nude or in highly revealing outfits, as stealth is not a priority.

Her relationship with Miko evolves from initial rivalry or envy into cooperation after the clan's destruction. Together, they combat common enemies, including the Shikima demons and rogue ninjas. In broader media, like the live-action film *La Blue Girl Live 3: Lady Ninja*, Yaku recruits her cousins Seia, Fuka, and Maki to aid in battles against the Shikima.