Mob kara Hajimaru Tansaku Eiyuutan

Kaito Takagi is such an ordinary high school student that he thinks of himself as a background character. Since mysterious dungeons appeared in Japan, he has worked as a low-level explorer, spending his days hunting slimes for modest rewards with no expectation of fame or heroism.

Everything changes when he defeats a rare golden slime and receives a legendary Servant Card. Using it summons a tiny valkyrie whose power is far beyond anything Kaito has encountered, opening a path he never believed he could enter. From that moment on, the explorer nobody noticed begins to discover that the dungeon's deeper floors might give him a chance to become a genuine hero.