The Weakest Summon, The Strongest Power

Sam Harper picks a Rank F succubus named Zythra on the day the students at his school awaken their Eidolons, and everyone in the hall laughs at him, because he turned down a Rank S Sangrifex to do it. He knows something they do not. Zythra has a hidden bloodline, and it can be woken. Sam arrived in this world from somewhere else and has a system that shows him things other people cannot see. He takes out a mortgage on property to pay for what it costs to activate Zythra's sacred blood. She evolves to Rank S with two attributes, light and dark, and unlocks a weapon form of herself. Sam also gets hold of the Bloodmist sword. The pair go through the academy's trials, kill a two-star beast in Swallow Forest, and put down a riot in a secret realm. Zythra reaches Rank SS and the whole school hears about it. Yue Wangchen and his father scheme against Sam, and the vice principal works against him too. Sam keeps raising Zythra, and beasts eventually surround the city.