Devour to Dominate

Freya Freeman is the best student at her school and the one everybody there watches. She performs a summoning and what she gets is Matthew Cross, a small skeleton who talks. Matthew was an eighteen-year-old boy until he woke up inside the bones, and the summoning was not something either of them planned. He comes with the Insatius System, which is rated SSS. Anything he chews through makes him stronger. The school decides the two of them are the most useless pair on campus and says so to their faces. The system picks that moment to speak up: "Ding. SSS-rank growth potential detected." Later a group at the school corners Freya with something she cannot get out of on her own. Matthew goes down into the dungeon and eats the boss at the bottom of it, and comes back up as the Skeleton King, with muscle on him now. He tells Freya to stop crying and says he will crush them.