Boku no Heya ga Dungeon no Kyūkeijo ni Natteshimatta Ken: Hōkago no Isekai Bōken-bu

Synopsis

After my parents disappeared, I, Tooru Suzuki, bounced around from place to place and ended up as a caretaker of a school dormitory in Tachikawa. The building is in disrepair, the student boarders are all weirdos, and of course, there are rumors about ghosts haunting it too. It turns out that believe it or not, the dorm is connected to another world! Although I don't have a single friend in class, I can always just go to the dungeon to slay monsters and level-up. There, I became friends with a slime, a female knight, and even a female sorcerer! Thus, I begins another day of after-school club activities in another world!

(Source: Takeshobo, translated)

Note: Includes chapter 0.