I Think My Friend's Going to Become the Demon King

Sacher Blythe, desperate to live a long and peaceful life, fails to avoid the future demon king, Phelan Exa Drozen. But Phelan knows more than he should…he recognizes Sacher as an "outsider”--someone from another world who knows the future and wants to kill him. So Phelan proposes a truce and Sacher agrees, hopefully preventing him from becoming the demon king. But this agreement keeps Phelan closer than Sacher would like...and their reluctant friendship is quickly turning into something more.

(Source: Tapas)