Basically they have known one another a while. Whether they grew up together or her family was/is close to his...we can decide on how they have known one another. He was gone for years in the marines and would see her sometimes when he came home. They could have kept in touch as friends when he was gone, depending on how close they were. They ended up together a couple years ago (or a year-ish, whatever works) and were together up until earlier this year. He's not very great at communicating and opening up (he has a touch of PTSD) and tends to put a wall up when he's stuck in his own head. He threw himself more into his work rather than talking to her about things. Eventually she just got tired of all of the working he did and lack of communication. They ended things but because of their past, they've tried to remain friends, but on his end it's been hard. He can kind of be an asshole and isn't cool with her flirting or dating though he has no right to be pissy about it lol. She's the only relationships he's ever had.