Cecilia, as a noble, would naturally have a retinue with a knight or two acting as her personal guard. Like Claire, and maybe Aurich. Ashton too if he no longer wishes to be an enemy unit recruited to the party during battle. Besides knights she would have other retainers or servants or just friends loyal to her cause like Seth and Edmund (hidden personal goals aside). Then there's the Advent mercenaries which she could contract Ysena for their services (if needed, I may buff the party with some allied NPCs to even the odds against a large enemy force).
Independent characters like Valerie and Maribelle (or Aurich, I'm not decided on him yet) can join Cecilia as a personal choice or oath to serve, or join Ysena's mercenaries and accomplish the same goal of joining the main party. The two (or three) can also be in a group themselves as either a wandering troupe that decides to put themselves between the enemy and the other players or villagers, or hired/forced soldiers working for the enemy.
Also, if you guys have any friends who'd like to join, feel free to invite them so we can add to the party size. Or later, after we've streamlined the game play and get used to the process we may start inviting newcomers (appropriately leveled) to the party in different chapters. These could be other people on Elliquiy or, if need be, secondary or even tertiary characters for some of us if we want or need them.
As a character, I had in mind an urban knight of sorts for Aurich. A knight of a free city rather than the landed, big-estate type we normally think of, he spends his free time as a merchant and bureaucrat rather than hunting. He fashions himself an up and coming tactician and statesman. As stated he could either be a guard or friend of Cecilia or a passerby that joins the team as a sellsword or oathtaker.
Now, building off Latooni's suggestions, the prologue will open with Cecilia forced into rebellion. Her manor, a small gift her father left her since she couldn't inherit much else between her siblings and their greed, has come under attack and overrun along with the nearby town and villages. Cecilia's forces regroup and, with the help of others from the towns, villages, and even passing mercenaries, fight back to retake the small estate. Initially caught off guard because she wanted to keep negotiating for temperance and kindness from her siblings, she is then branded a rebel once the small force her brother and sister sent fails to kill her. Even if she never wanted it, she becomes a figurehead for the new rebellion.
This can satisfy a lot of requests from various players on how they want to join the party - Ysena's mercenaries can either be passing by and decide to help, or perhaps secretly hired by one of Cecilia's followers who foresaw this treachery and they arrive in time to join the counterattack. Maribelle can be in one of the houses or villages taken over by the enemy who didn't expect Cecilia's small party to find help and attack so soon. The region can be a sort of crossroads for any individualists to likely pass by. Ashton can see the injustice in this one-sided battle that feels little more than an assassination attempt, and so on and so forth.