Kono Hoshi Saigo no Ren'ai o.

Five dragons have appeared in this world. Without any way to defeat them, human civilization is gradually being destroyed. The people prayed for a hero, so the Financier of Chaos appeared before the last princess with the ability to slay dragons... Takaku Tatsuma. Humanity made two miscalculations -- the "princess" was a 16 year old male, and the hero... their chosen hero... had already gone completely mad.