Tomari Seki

Tomari is a young girl in fifth grade of elementary school. She first met Potemayo while chasing Guchuko after she stole some chestnuts from her. She later ran into Mikan and her brother Yasumi, whom she takes a liking to after he gives her a hat that Mikan had originally given her brother. Tomari is a tomboyish girl who rarely wears female clothing and has very short hair for a girl, which makes her look like a young boy; it came as quite a shock to Mikan and Yasumi to find out Tomari was a girl.
(wikipedia)