Mei Sunohara (春原 芽衣)
Background
Mei is Youhei's younger sister who loves Youhei as a brother and lives in the countryside. Mei is a smart girl and likes to see new things. She is a fan of Yusuke Yoshino, an older male graduate from Tomoya's school, who is a member of a band. Worried about her brother's situation, she calls Youhei, but instead of her brother, Tomoya disguised as Youhei tells her to come.
Mei's personality is a direct opposite of that of her brother, Youhei. She can be very nosy about things, like trying to further Tomoya's relationship with Nagisa by becoming their cupid angel or learning Youhei's current situation.
(Source: Clannad Wikia)