Race: Night Sparrow Youkai
Ability: Night-Blind, Confussion Song
Age: Unknown
Occupation: Grilled lamprey Vendor
Location: Forest near Human Village
Mystia is a little Night Sparrow with an egocentric personality - at least she sounds arrogant - that enjoys infusing confusion and terror into the hearts of humans using her ability: a song that creates confusion in those who hear her. Her song seems to attract nearby monsters as well. She roams a road near the Human Village that Keine tries to protect, in search of humans to frighten. She is indeed feared by the locals as people around her become blinded by darkness.
She gets "eaten" by Yuyuko of the Ghost team in Imperishable Night's story mode, making her a popular "joke character" as she's usually pictured as food. Even ZUN mocks her in his spell card comments.
She hates Yakitori (grilled chicken on sticks) because she is a kind of bird. So she started a grilled lamprey stand instead of a Yakitori stand. (It was believed that lampreys had eight eyes and are able to cure night blindness.) She uses this as part of a con-act: she will travel with her stand at night, use her power to make unwary travelers night-blind, then sell them grilled lamprey, and slowly remove the effect as they eat it, claiming that the lamprey was what cured them.
During the events of Phantasmagoria of Flower View, Mystia aimlessly wanders around singing, hoping to catch and eat humans attracted by the flowers. Instead, she attracts (or rather, annoys) the other protagonists and ends up fighting each in turn.
Even though her songs are meant to scare and confuse humans, in many fan works she is depicted happily singing with a graceful voice, sometimes viewed with a microphone. It is also common to find her alongside the Prismriver Sisters in fan works, where the four makes up a band in which she serves as the main singer.