Shinkai Tsuki

The protagonist's little sister. Due to family circumstances, she's mature for her age and is quite assertive. While she respects her older brother, she doesn't want him to keep overexerting himself for her sake, so she firmly objects whenever he does something wrong. Though she acts her age on rare occasions, she always has her guard up. Lately, she's started to think her brother is acting gloomy, which the protagonist perceives as a sign of her having reached her rebellious age.