Mystia Lorelei is a type of youkai called the night sparrow that can bewilder people by singing. She can also give people night blindness, so that they cannot see their surroundings.

If you go to Eientei to have your night blindness treated, they will give you a diagnosis and suggest that you eat unagi (Japanese eel) to treat it.

Mystia runs a grilled unagi stand which is very popular in Gensokyo. She started the stand to try to eradicate yakitori stands selling grilled chicken skewers.

Even if Mystia's ability makes you night-blind, don't panic. As long as you eat some grilled unagi at her stand, your eyesight will return to you (or well, Mystia will return your eyesight to you). Making people night-blind so that they will eat at her stand is definitely good for her business, but it is unclear if Mystia herself realizes this.

If you are walking alone at night, you might run into the night sparrow, so you should ask Mokou to join you. If Mokou is with you, the night sparrow will be gone in no time. If you ask Mokou why, she would tell you that she doesn't know, but it might be because she's a health maniac running a yakitori stand.