I vaguely remember people theorizing or data mining that the 4 new characters are mercenaries. I am wondering if the side route is an alternate route you can take where you lead the Knights of Seiros doing stuff they did in the main story? You’d be Byleth, and be an influential person in Jeralt’s ear. He’d have a hand in hiring the 4 mercs, and you’d get Alois, Shamir, and Catherine. So you’d start the game with 9 units, like the other routes.

I know support convos with other Lords were data mined, so that could be a wrench in this idea, unless this were the key to the golden route? Haha, and this point I’m just using conjecture more than I was at the start but it’d be interesting if the most successful and “good” route comes from Byleth working with the elite Knights of Seiros instead of a House.