Hitoribocchi no Isekai Kouryaku - Vol. 13 Ch. 140 - The Ultimate Protection

[MangaDex]

 

[Xeroxcopy4free]

looks like the angel version of skeleton key in elder scroll games

 

[Otherworlder]

I like how even after all this time the MC stubbornly insists on getting absolutely everyone's names wrong. Well, except he's finally started calling Angelica by her name. Not only do they answer to the wrong names now, but they've even caught on with others, like how all the girls call Invisible 'Stalker-chan', or how Prez referred to one of her classmates as 'Poster-chan' several chapters ago.

Using a magic key that was never shown before, even though he's had it since the Great Labyrinth, is kind of an ass-pull. It would have been better, and funnier, if he just casually picked up both girls and teleported inside.