Niwa Kuon

Likes: Aiko
Dislikes: Nothing much
Hobbies: Visiting the zoo

Kuon is the new assistant homeroom teacher to Satoshi's class. She is mysterious and reminds Satoshi and the others of Yui, and tries to get Satoshi to like her by doing strange things, such as buying him his favorite food and having it be delivered to him by a helicopter. She is also Aiko's older sister, whom she calls "Ai-chan" and treasures very much.

Spoiler

Kuon has an abnormal brain and white hair, making her the "perfect" genius. People have constantly pushed her to exert her brain and life energy which led to her having the body of a 90-year-old at the age of 24. This is shown in the onsen event in Chapter 5, how she is abnormally skinny, her body being almost nothing but skin and bones. The watch on her wrist shows her remaining lifespan.