Sei: is one of Ichika's tutors, and a high school student. He instructs her in grammar, literature, and social sciences. He is introduced as the twin brother of Kai. Of the two, Sei usually seems to be the calmer, gentler one. Ichika has developed romantic feelings for him.
Kai: is Ichika's other tutor. He teaches her math and science. He is introduced as the twin brother of Sei. Of the two, Kai seems the more extroverted one and more physical in nature. He is also in love with his twin brother Sei.