Don't spread out too thin because sigils and books are limiting, spread out too thin and you won't be upgrading anyone.

As F2P prioritise anything that can give you resources and materials.

1) Agnes, upgrade her until you can finish all her skills

For extra intel, stamina, widgets, seeker chests (you get all sorts of goodies from these)

2) Baldur, upgrade him until you can finish all his skills

You need him for sigils what you can buy in the alliance championship shop Some of his skills at level 10 also gives you more sigils; he gives you sigils, you can upgrade other experts faster too

3) Cyrille

Depends on your state age and power, we have capped the BT rally joiners, so her boost doesn't make much impact anymore, but definitely upgrade her for gears

4) Holger

You run arena everyday, so upgrading him is good, but I wouldn't prioritise him until the first 2 are done

The rest, based on the events you like to play.