So. Story is a (very low) fantasy - pretty much just fictional countries and people, no magic or dragons. One of my leads is a past-his-prime general, who had quite the wild youth, and who is sent to negotiate with another country's young queen. His own king secretly hopes that, not being a diplomat, he'd fuck up spectacularly and get killed, thus giving a good casus belli (they want the queen's country). Our general has a very close friend/confidant who has been by his side for decades, and who accompanies him on his mission. Things go rather unexpected - not only does the general not fuck up, him and the queen end up falling in love and he stays. The war eventually happens anyway, but now they stand against the asshole king. The war goes in their favour for a while but eventually things take a bad turn. At some point, though, I want the friend to desert his general and go to the enemy (who receive him in a rather civil manner). But why would he do this? He regrets it pretty soon, especially when his former general loses the war. He'd need a damn good reason to desert in the first place, since he loves the general deeply - so much that he can't handle the guilt. He's definitely a positive character, so it's hard to find him a reason to leave.