Well, if the Lady insists...
I've mentioned a few times some thoughts on a 'Dear John' romance- or rather, a companion whose character arc centers around the prospect that his wife is cheating on him while he's away saving the world. (Technically this doesn't require marriage or a male companion, but that's the default I'm using based on the history that gives it the name.)
A Dear John arc would center around a perfectly decent and good person whose initial resistance to romantic advances is clear- he's in a happily committed relationship, and even though he regrets being away he's resolved to return home after all this is done. Absence makes the heart grow fonder.
Except... one day, something changes. He's distracted. Irritable. He doesn't want to talk about home. And after investigating, you find why- a letter that begins 'Dear John,' and ends with the revelation that his former beloved wishes merely to be former. Distant travels, deposits through the Dwarven merchant guild, and brief visitations aren't enough to sustain a household or raise a family. She wanted more than to wonder if she would be a widow, and someone else has offered her that.
It's hard, it's painful, and though there's a clear culprit there's no clear blame, even if the wife is confronted. And why should there be? Perhaps, a great guy as he is, as committed to the world as he was, he wasn't as committed to his family as he vowed to be. She doesn't have to be a villain. She doesn't have to be unsympathetic, even. Perhaps how she did it was wrong, but what she did was all too reasonable. She pursued her happiness with someone who was there for her, rather than someone who loved her but wasn't.
Maybe there is an option for reconciliation. Perhaps, rather than cheating, this was the prospect of divorce. Or perhaps her new love also died. You could even frame it that the letter was forged, the plot by a jealous would-be suitor to break the two apart by John's reactions.
There are many ways it could be framed, and many ways it could offer the PC to role play, romance arc or not. Say you're just his friend, or comrade. Do you play the sympathetic friend, reminding John there are other women out there and that he should move on? Do you play the marriage counselor, and do all in your power to reconcile them? Do you hate and condemn the betrayer, or understand and forgive if not approve of what she did?
Or, if you love him yourself... is this your chance? Do you comfort him, let him know that there is someone close to him who still loves him and would never abandon him? Would you go so far as to subtly sabotage any chance of reconciliation, just so that he is free for you? Or do you pine chastely, but do the honorable thing and try to make him happy be reconciling him with the woman you are jealous of and who doesn't deserve another chance from him?
That's a romance arc character I would want to design.