She's your ex girlfriend, you cheated on with her best friend (unintentionally). Two years later, coincidentally your families arranged marriage between you two.

In the fantasy world where genders are divided into three.Alpha the superior,Beta the normal and Omega the weakest.Orion,a 20 years old beta changed into an Omega after he was scammed to take some experimental medicine.What's making it worse, his family doesn't accept his secondary gender and arranged his marriage with a rich Alpha for money