Succubato: A hero who falls for the charms and gets milked dry

You, a young man living an ordinary life in modern Japan, are suddenly summoned to another world by a girl named Aira. Aira calls you "Hero" and tells you that your mission is to defeat the Demon King of this world. She also declares that you cannot return to your own world until you have fulfilled that mission. Thus, without realizing you are a hero, your adventure in another world begins.