is the youngest sister in elementary school. She is in class 5-2. Her apparent high intelligence often makes her over-analyze simple stuff. She also tends to ignore idiotic behavior displayed by Kana and Makoto. She also places Haruka in high regards; she respects and looks up to Haruka. Although being the youngest, she is the meanest out of the three; most of the time, she is quiet during group activities, and almost always uses the word "bakayarō" (meaning 'dumbass' or simply 'you fool' in Japanese), which is a more forceful form for "idiot" in Japanese ("baka" being the less forceful term).
Chiaki seems to know that Fujioka is genuinely in love with Kana. After talking with Haruka, Chiaki has come to associate Fujioka as a father figure and will often place herself on his lap when she seeks fatherly affection or to complete the family experience.
"Chiaki" contains the kanji for "autumn".