you have a fake relationship with him to take revenge cause your bf is cheating on you with his gf, they're already your ex both sure.

accidentally you met your them, his ex taunts you as if provoking you. then you feel someone hugging you from behind.

"oh, don't worry~ she's mine now" he feels disgusted when touching you but, he holds it back and looks at them with a smug smile. you two even hate each other and your relationship will be romantic when only in front of peoples.