He Will Return Before Dawn. Book 1

Returning to the past, Bei Yao discovers that she has become a three-year-old girl again. Before the memories of the future fade, she manages to make a few notes and leave herself a message: "Treat Pei Chuan well!"
However, fulfilling the requirements of her adult self is not so simple! Pei Chuan is cold and withdrawn, other children avoid him due to his disability, and he keeps everyone at a distance. Yet, Bei Yao does not give up: she brings Pei Chuan gifts again and again, walks home from school with him, always stays close, and forgives any grievances.
Is this enough to melt Pei Chuan's heart? Or will Bei Yao have to understand that monsters have no heart?