I'm the Demon King!

From The Zero Alliance:

In the midst of a battle against heroes, the demon king, Rou is transported to a different world, where he ends up as a high school transfer student. When his classmate, Hitose discovers his true identity, she tells Rou that she wants to help him get back home. Although Rou thinks of Hitose as annoying and strange at first, they manage to form a friendship.