Mob kara Hajimaru Tansaku Eiyuutan

Kaito Takagi, a low status high schooler, your typical Mob character so to say, was just a normal everyday explorer who hunted slimes every day in the dungeons that appeared in 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, he obtains an extremely rare item worth hundreds of millions as a drop, a servant card that can summon mythical beings!! After he makes up his mind and summons it, a warrior maiden with unparalleled beauty appears. Rise above being just a mob explorer Kaito! A modern battle fantasy story has begun!

(Source: Hobby Japan, translated)