A Sidey
A Sidey is a side girlfriend or boyfriend, the one you date while you're already seeing someone else. They’re the extra person you can’t get rid of.
He’s a Sidey, and I’m dating both of them at the same time.
She’s my Sidey, and she’s always showing up when I don’t want her to.
He’s a Sidey, and I can’t get rid of him no matter what.