Yuu Nikaidou (二階堂 悠, Nikaidō Yū) is an former employee member of Easter in Season 1.
In order to find the Embryo for Easter, he joined Seiyo Elementary and became teacher.
His childhood dream was to become a robot engineer, but when his mentor, he lost sight of his dream and became a twisted person after he broke his heart's egg.
He recovered with help from Amu and Su. He resigned from Easter Company at the end of season 1 and officially become the teacher in charge of Amu's class in the following seasons. A running gag in the show is that he calls Amu Himamori, meaning "leisure time".