I tried to keep this as spoiler free as I could but with enough information to avoid loosing or not getting a character.
If you don't want to be spoiled but don't want to take the risk of loosing a character it's for you.
Character are listed in the chronological way of their first appearance or cause of their appearance.
I can notice you that Alette, Rook and Yver are mandatory for the last fight, so I would advise to up them, specificaly their armor break. If you want to keep Alette innocent, you can still make her fight against dredge.

Chapter 1 and 3 are about the eastern caravan, all the characters present in the caravan at the end of chapter 3 will join you again for the final fight except for Fasolt as far as i know.

Gunnulf
  • Here at the beginning
  • Leave you in chapter 3 if you don't let the cart go
  • Join you after Einartof if still their
Eirik
  • Here at the first fight
  • Join you in chapter 3 if you help him until the end in chapter 1
  • Can leave if you let him in the same chapter (he wants to go back to strand when you send people this way)
Hakon, Mogr
  • Join you when you leave strand
  • Will help Yver on the bridge if you don't
Rook,Alette
  • Will stay in the group until the end
Iver
  • Join you when you arrive at Skogr
  • Leave after the first bridge fight
  • Join you if defend the bridge a third time
  • Join you anyway when you leave Einartof
Egill
  • Join you when you arrive at Skogr
  • Leave if he fights right away (might exists a way to keep him but didn't want to risk it)
  • Leave near the end if Enef joins you
Trygvi
  • If you have backed or pre-order will join you if you send Egil and Alette away

Didn't backed or pre-order so i can't know more
Oddleif
  • Join you after you meet the chieftain
  • (might die if she goes alone rob the horn in the final chapter)
Hoggun,Moggun
  • Join you if you side with them ( maybe if you don't side with the villagers too)
  • Don't know if this is a bug but Hoggun died for me when i tried to go through the dredge instead of taking the other road ( this was my 3rd playthroug so didn't read :-( )
Ludin,Bersi
  • Join your roster after you encounter the first dredge group
  • when meeting with Fasolt you can send them away
Ysra
  • Join you with Ludin and leave you with him
  • Leave you if you set the fire at the forest in chapter 3
Griss
  • Tries to join you when you approach the village in chapter 3
  • Tries again if you save the village
  • (Might help Yver on the bridge if you don't and if you let him join)
Fasolt
  • Join you after the village destroyed by the dredge in chapter 3
  • Join you after Wyrmtoe
  • Will help Yver on the bridge if you don't
  • Leave you at Einartof
  • You can fight with him twice trying to defend the bridge
Enef
Nasty one, not sure i know everything about him
  • Will try to or will join you if don't join Ekkil at the end of chapter 2
  • Will leave you near the end
Ekkil
  • Will try to join you if Enef joins you
  • (Might try if you didn't join him at the end of chapter 2 even if you don't have enef)
Nid Lvl 1 but with a lot of point to add right away
Tricky one
  • When Oddleif is killing birds, she might get the idea of training archer,
    If you let her Nid will join you a bit after Einartof
Krumr
  • Join you at Wyrmtoe
  • Leave you after at Einartof
  • Join you again if you don't leave Einartoff right away
  • Leave you near the end (another moment that the Enef event)
  • Join you again a bit after
Eywind Lvl 5
  • Join you at Einartof
Another one i don't remenber the name :-) Lvl 5
  • If you go help the house on the last step before the end will join you
  • you can leave him just after
  • Leave when you arrive at the final step ( there might be a way to keep him )