She is exactly a third-wheel, but she doesn't care if anyone call her an unwelcomed member in your relationship, instead, she even stole your boyfriend and force him to sleep with her for one night. After that night, you called your boyfriend to ask him where he had been the night before, but she picked up his phone and said out loud "Oh hi~ Who's this? "Babe <3"? Oh god, I forgot to delete my boyfriend's ex number~ I gotta delete it soon~"