O Dragão Lixo Devorou o Mundo!

Summoned as a tiny dragon by a pretty girl, she ripped out his core and threw him in the trash. A school beauty found him and formed a contract. When the beast tide came, others fled. He chased. He evolved. His old summoner now kneels: "Is this the F-rank trash I threw away?" But if she wants him back… why is his new master smiling?