Kuzuha is Yuuta's younger sister who is in the first year of junior high school. She knows Yuuta very well, especially how he acted during his Chuunibyou days in middle school as Dark Flame Master, though she always considered him embarrassing. Kuzuha got used to her older brother's delusions since she was young. Every once in a while she would ponder if her brother's acts were actually cool before immediately dismissing the thought as Yuuta would spectacularly embarrass himself yet again.
(Source: Chuunibyou demo Koi ga Shitai! Wiki)