Riku was initially presumed deceased before the main events on Island 6 but was later found alive there by Wakaba. She had separated from her sisters to independently investigate the world's decay, concentrating on the connection between Kemurikusa leaves and the predatory akamushi infesting the environment.

Her personality blended an impish, playful nature with deep caring for others. She expressed longing for physical contact with her sisters during isolation, reflecting her name’s meaning—"agony of separation" in Japanese. Among the six sisters, she stood out as the most clumsy and careless, traits her siblings often cited when describing others. Despite this, she displayed bravery and a fierce adventurous spirit, habitually exploring and excavating new territories to support her family.

Physically, she wore a hooded black dress with a red scarf and tied her short magenta hair into pigtails. Sharp teeth were her distinctive feature. Her specialized power was an acute sense of touch, granting heightened tactile perception. This ability made her adept at detecting akamushi threats and traversing perilous landscapes.

Her story ended in a heroic sacrifice during the group's mission. After death, her red scarf passed to Rin, transferring her sensory ability. In the final battle against the Red Tree, she briefly returned alongside other deceased sisters to deliver critical aid before vanishing permanently.