Mob kara Hajimaru Tansaku Eiyuutan

Follows Kaito Takagi, a high school student with a mob character and low status. He's an ordinary explorer who hunts slime everyday in dungeons that appear in Japan. One day, he encounters a golden slime that he has never seen before. Baffled, he still manages to defeat the slime, and get an extremely rare item called the "servant card," which can summon mythical beings, and has a value of hundreds of millions. Once he is decided, he is also able to summon Valkyrie, a beautiful warrior maiden. Kaito starts his adventure to rise from being a mob to a hero.