Hiyama Kiyoteru is a main character in the series. He is the sensei of Yumi's class.
It seems that he has a semi teacher-student crush on Nakamine Yumi. Further info on Kiyoteru is on the Vocaloid wiki.
Before Yumi came, he is a strict teacher. but ever since Yumi's appearance, he seems a little softer especially with Yumi, Teto and Momo.