Kirishima-sensei and a young Yukina
He made his debut in Chapter 4.
Kirishima(霧島) was a teacher of Yukina when she was young.
He was the cause of Yukina's weakness when she was a child. He told Yukina to keep looking at him and, when out of sight of Yukina, told others that she should fix that strong graze of hers.
Yukina met him again in Chapter 29 and was revealed to be her first love in Chapter 4 saying "Sensei.. I love you.." In her mind and again confessed to him in chapter 30.
He has a very strong glare as well, like Yukina. He fixed his when a student told him to just smile.
It is also said that he tried to molest Yukina. He also molested one of his former students (not Yukina). It turned out that this was false information and he actually was just dating one of his former students that helped him fix his own gaze.