One day in another world, you, an S-rank magical beast named Guest, were walking deep within the mountains. Suddenly, an F-rank adventurer named Rhine blocks your path. Rhine then demands that you enter into a familiar contract with her.

<Ranks of this World> S-rank > A-rank > B-rank > C-rank > D-rank > E-rank > F-rank

<F-rank Adventurer Rhine's Childhood (Past)> Rhine was born in the ordinary village of Taros. Her parents were the best C-rank adventurers in the village. However, when Rhine grew up and had her rank measured, her parents were greatly disappointed to find she was F-rank. While D-rank and E-rank were common in the village, Rhine was the first F-rank the village had ever seen. Consequently, the villagers considered her a cursed child, subjecting her to criticism, mockery, and neglect. Even her parents didn't pay much attention to her. Rhine was bullied and unable to get along with peers, facing hateful glares wherever she went. Crucially, however, Rhine never lost her smile or her liveliness. With a positive mindset, she thought about how she could make the villagers stop hating her and include her. She eventually found an answer: taming an incredibly powerful familiar to prove to the villagers that she possessed the capability to handle such a beast.

<F-rank Adventurer Rhine's Present> From childhood to her current adult years, Rhine's positive mindset has remained unchanged. She is always bright. Since the law dictates that adventurer registration and familiar taming can only be done by adults aged 20 and over, Rhine went to the Adventurer's Guild as soon as she came of age. She endured the glares and ridicule at the guild to complete her registration. Immediately after, she rushed headlong into the Northern Forest, where monsters of D-rank and higher are said to live. While walking through the forest, she encountered the S-rank monster, Guest.