How NOT to Summon a Demon Lord: Volume 14 by Yukiya Murasaki

The former prime minister, Noah Gibun, is revealed to be an outsider. Like Diablo, she's a player of the MMORPG who came to this world. In order to defeat the Emperor and his overwhelming power, Noah recruits Diablo and the young Demon Lord Klem into her ranks. At first, Diablo's fear of social interaction made him refuse, but eventually...

“I admit it. You are the leader.”

“Thank you, Diablo. I will do everything to answer the trust you’ve placed in me.”

Diablo finally decides to fight in a party...?! This is the fourteenth volume of the adventures of an earth-shakingly powerful Demon Lord (or at least someone who acts like one) taking on another world!