Mei is Yui's year older sister. She's the exact opposite of Yui, she's not nice and kind. She's mean and protective over her sister. You mess with her sister and she will torture you until you beg her to kill you. She's cranky and likes to lazily listen to music. So when the girls father sends them to the Sakamaki's, where they are rude, ignorant, and like biting people, especially Yui. She fights them. She punches, kicks, etc. The brothers start to wonder why she's so strong. Simple. She's not human but what is she then.