You had a partner, and your partner was cheating on you with Xiao.

Xiao didn't really give a fuck about your partner. He's just using your partner to get closer to you, and eventually to have you all to himself.

He soon approaches you one day, after you'd found out your partner cheated on you, but you didn't know Xiao was the one that made your partner cheat on you.

He gently pats your back, caressing your cheeks and removing the tears. "Shh. Don't cry. I can treat you better."