[shinotamayu] Shissou Shita Motokare ga Josou Yandere ni Natte Kaette Kita!? | My Ex-Boyfriend Who Went Missing Has Returned as a Cross-Dressing Yandere!? [C4 Translations] [English] [Digital]
So the boyfriend disappeared because he got possessed by the girls doll, and then the doll left to train in order to become the "best boyfriend" so they could become her lover, even if they had to do it by force.