Manatsu Kuroki appears as a mirror spirit and dream entity interacting with Ichika Tachibana. Her existence originates from Saya Kogure, the original mirror spirit, who formed Manatsu specifically to oversee Ichika's experiences involving the Djinn. This creates a complex bond where Manatsu perceives Saya simultaneously as a mother and a sister, yet resents Saya's involvement in causing Ichika pain through the ritual.

Personality-wise, Manatsu presents a stark contrast to Ichika's reserved, anxious, and serious nature. She embodies a carefree, lively, and assertive demeanor. Despite these differences, they share overlapping interests and a mutual fear of ghosts. Manatsu exhibits excitability and openness but is also prone to boredom. Her intellect and capacity for deep care often exceed initial impressions, revealing an essentially selfless character.

Her defining commitment centers on Ichika. Manatsu feels profound guilt over Ichika's suffering during the Djinn trials, frequently expressing a desire to alleviate her pain. This drives repeated self-sacrificial acts: preventing Ichika's suicide, shielding her from a tsunami (an act punished by Saya), and ultimately taking Saya's scythe strike to save Ichika during the ritual's climax. This final sacrifice reverts Manatsu to her original state—a shard of the mirror. Subtle narrative cues suggest romantic feelings underlying her devotion.

Her concluding arc unfolds in the OVA, "Shotō no Futanatsu." Manatsu temporarily reappears as a spirit one week before Ichika's December birthday, enabling a final reunion. After spending Christmas Eve together, she bids farewell and returns permanently to the mirror by morning. This departure coincides with Kai's similar return, leaving Ichika and Sei to continue their lives.

Physically, she is depicted as petite, standing 142 cm tall, and often seen in distinctive attire like flowy dresses or pigtails. Her name, written with kanji meaning "midsummer," reflects her initial manifestation during that season.