I have been curious what happens to the character not selected ever since there were some leaks about this very issue so SPOILERS ahead!!!!
The leak talked about them being siblings and that they would be the Primary antagonist. Leaks aren't always correct, but these ones in this leak have for the most part turned out to be true. What if it isn't so much that they are the main antagonist, but perhaps one of the mercenaries sent to kill us. It would be interesting if they aren't siblings and if we can recruit/romance them. Ubisoft has perhaps intentionally skirted around this topic but It is something I find could lead to a whole lot of cool possibilities in Assassin's Creed Odyssey.
I have a few other theories in this video.
but what do you guys think? Will they even be in the game if we don't select them?