Manatsu Kuroki (黒城 舞夏 Kuroki Manatsu) is a girl that Ichika Tachibana met in dreams and a mirror.
Manatsu seems to be the polar opposite of Ichika Tachibana in terms of personality. When Ichika is hesitant, Manatsu is carefree and assertive; where the former is reserved, the latter is cheerfully outgoing. She also has striking similarities to Ichika, namely, she knows and loves many of the same things Ichika does, and she shares the same fear of ghosts.
Manatsu is very excitable, but she also easily bored. She's usually very open who has a happy disposition though when it comes to Ichika, she gets serious about the well being of her dear friend. She is smarter than she seems and is very caring. She has offered to sacrifice herself for Ichika several times.
Manatsu cares deeply for Ichika and feels guilty about making Ichika go through the experiment. She often mentions how she feels terrible that she can't even cry over it and wishes she could do something to save her. As Ichika takes on more of the powers of the Djinn she worries endlessly over her friend and how the outome will be as she watches Ichika become taken over by hate for everything around her.
Manatsu nearly sacrificed herself for Ichika several times. (Such as when Ichika nearly commited suicide and when she stopped the tsunami from hitting the shore and being punished by Saya). It is never said outright but is hinted that Manatsu actually has romantic feelings for Ichika.
Near the end, Manatsu does in fact sacrifice herself for Ichika, then returning to the shard of mirror. In the OVA she comes back and spends one more night with Ichika before disappearing forever. Manatsu is a young girl that Ichika met in dreams and a mirror.
Saya is considered Manatsu's mother and sister in a sense because both of them are spirits of the mirror. Saya was the original though who made Manatsu to watch over Ichika. Manatsu resents Saya to a certain extent for forcing her to hurt Ichika.