Mob kara Hajimaru Tansaku Eiyuutan

Kaito Takagi is a low status high schooler. A background mob character, you might say. He is just a low-level explorer who hunts slimes every day in the dungeons that appeared all around Japan, earning pocket money while admiring his childhood friend, the class queen, from afar. One day, a golden slime he had never seen appears before him. After he defeats it, it drops an extremely rare item, worth hundreds of millions, a card that can summon mythical beings!! After he decides to activate the summon, a warrior maiden with unparalleled beauty appears.
Rise above the mob, Kaito! A modern battle fantasy story has begun!
(Source: Hobby Japan, translated)