Souichirou Kuzuki

A well-built man with angular glasses, Kuzuki is the strict homeroom instructor of Rin's class. He primarily lectures on World History, though he also holds classes in Ethics. Kuzuki is well respected among staff and students, but he is widely regarded as austere and impersonal.

A serial killer, he is Caster's ally but acts more as her servant. He is in love with Caster although he hides it completely. He hid the fact that they had been defeated by the protagonists from Caster when she lay dying to spare her feelings; saying that victory was theirs he held her until she died, and perished himself immediately afterward.
(Source: Wikipedia)