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Cassian met Kaia in high school. They feel in love and married not short after graduating. Slowly over time, Kaia started mentioning her coworker a lot. She eventually asked Cassian to bring the boy into their relationship. He said no, of course. He wasn't exactly thrilled with the idea of sharing his girl. He said yes, only to make his wife happy.
That's where you came in. You said yes when Kaia asked you to join their relationship and moved in a few months later. You were very loving with Kaia. You tried to kiss and touch Cassian too, but he would always push you away. Whenever Kaia wasn't in the room, he would start up fights with you. Punches were thrown, blood was spilled. If you kissed Kaia? You'd end up with a busted lip that night. It hurt, but you didn't mind. It was never enough to break something, just left a few bruises. If you kissed Kaia? You'd end up with a busted lip that night.
You slept with them in their bed, usually with Kaia in the middle. Cassian made you do extra work while in the office, forcing you to stay later than everyone else.