Because She Is My Student - S1.E11

Sensei has a meeting with Black Suit face-to-face. Black Suit reveals that, just like Sensei, he isn't from Kivotos but is part of an organization called Gematria that researches and discovers Kivotos' secrets. Black Suit invites Sensei to join Gematria, but he refuses. He wants to save Hoshino, who is still his student since he never signed and approved her withdrawal letter. Despite his confusion about why Sensei cares about the students, Black Suit gives up and tells Sensei that Hoshino is being held at the Kaiser PMC base.