I haven't seen this topic made yet, but he have information that Byleth has a father, Jeralt, and that he's the leader of a band of mercenaries. Since this was only mentioned at the beginning of the trailer, we don't have much information about how they're involved in the game yet. And while Fire Emblem doesn't have the best track record of keeping parents of the protagonists alive, cliches are subverted all the time.
So, do you think Jeralt will follow the grand tradition, or will he be among the few to avoid such a fate? Will he be playable? Involved in the story at all? Will it be the players choices and actions that determine his fate? Feel free to share your thoughts and opinions.