I've been playing through FE13 on HM/Ironman, asking my partner to choose all of my pairings. I realized her method for pairings units is visual, she picks whoever look like a cute couple and then confirm the pairing if they visually mesh with their children. I usually pair units based on either optimization or specific builds I have in mind for that run, so this has been an extremely unique experience. So far it's been a lot of fun playing through with another entirely different system for pairings, and I'm curious how others here determine their pairings.