DUNGEON DE NARIAGARE!: OSANANAJIMI KARA NO KIRAWARETE GOBLIN NI SAEKATE NAKATTA ORE GA, DUNGEON RULER NO SHIDOU WO UKETARA TSUYOKU NARETA NO DE IMOUTO TO MUSOU SHIMASU

It's been 30 years since dungeons appeared in Japan. Kyoichi passed the entrance exam to the same extremely competitive high school as his childhood fiancée, Tsubaki, but rumors of him kneeling before a goblin made him the target of teasing in class. He explores the dungeon in an attempt to turn the situation around, but is defeated by a group of goblins. Thinking of his younger sister, Koyoi, he prepares himself for death... when he is saved by the Dungeon Ruler, Kyoji. This encounter marks a turning point in Kyoichi's fate. Get stronger with every battle! The story of conquering unrivaled dungeons with your sister begins! (Source: Takeshobo, translated) — Description from AniList community (anilist.co).