Aiko Saitou

Aiko Saitou is Takeru's elementary school teacher and Nagisa's older sister who is jealous of Ika Musume's popularity with her students, as she was once popular with them before she came into view.

Aiko is shown to be very kind towards her students, and was very well respected. But if someone wedges their way into her student's respect she becomes very jealous. Her envy the total opposite to that of her sister, who is terrified with Ika Musume.

(Source: Shinryaku! Ika Musume fandom)