The game

Kingmakers factions

The Gameplay Deep Dive and the developer blogs both put you on one side: Owain Glyndŵr’s Welsh rebellion against England. The enemies named are English barons and nobles, and Scotland appears as ground you conquer, not a side you pick. The developers have said they want new factions after Early Access, without saying whether you could play them. A choice between England, Scotland and Wales has not been announced.

One side

You fight for Wales. No playable English or Scottish side has been announced. What would change this: a developer post or store text naming a playable faction other than Wales.

A dense crowd of soldiers with red tunics and shields charges through a village of timber-framed houses.
A crowd of soldiers in red charging through a village. Frame at 7:58 in the Gameplay Deep Dive, tinyBuildGAMES.

The sides, as the developers describe them

Your side

The Welsh rebellion

In Kingmakers, you travel to 1403 to help Owain Glyndwr transform a Welsh rebellion into an empire.

Redemption Road. Said in the tinyBuildGAMES video: Kingmakers - Gameplay Deep Dive

The enemy

English lords

English barons and nobles will throw massive armies at your settlements.

Redemption Road. Said in the tinyBuildGAMES video: Kingmakers - Gameplay Deep Dive

Scotland

Territory

As you expand your conquest across England and Scotland, you’ll earn experience to permanently upgrade your character class

Redemption Road. Said in the tinyBuildGAMES video: Kingmakers - Gameplay Deep Dive

The missions point the same way. The Deep Dive’s examples are besieging an enemy castle, hunting down an English spy and escorting Welsh noblemen, all in service of the rebellion.

A mission might task you with sieging an enemy castle, chasing down an English spy, or escorting Welsh noblemen through your territory.

Redemption Road. Said in the tinyBuildGAMES video: Kingmakers - Gameplay Deep Dive

Who you are in all this

Not a Welsh lord. You play someone from the future, sent back to change how the rebellion ends. The third developer blog puts it plainly:

As an American mercenary, you are sent back to the 15th century to aid Glyndŵr’s rebellion and restore the original timeline.

Redemption Road. From Steam announcement: Kingmakers Dev Blog #3, Picking a Fight with History

That timeline, and why the game’s version of history has Wales winning, is on Kingmakers story and setting. In co-op, every player is on that same side, each with an army of their own; see Kingmakers co-op and multiplayer.

New factions, wanted for later

Post-Early Access, we want to add new factions such as the Scottish or French, have epic castle sieges, deeper troop officer systems, and maybe even a PvP mode.

Redemption Road. From Steam announcement: Dev Blog #1, Meet the Troops You'll Command in Kingmakers

Two things that sentence does not say: whether those factions would be playable or would be new enemies, and when they would arrive beyond after Early Access. It is a wish list from a developer blog written before the delay. Where it sits among the other plans is on what Kingmakers Early Access includes.

Claims to be careful with

No source: Choose to fight for England, Scotland or WalesStated by Wikipedia, without a citation, and by a GamingBolt feature. Nothing the studio has published says it.
No source: A Welsh campaign only in Early Access, England and Scotland later as playable factionsPassed around on Reddit as something a developer wrote on Steam. No such post turned up in the Steam announcements or the game director’s forum replies.
Open: Multiple endingsClaimed alongside the faction choice in the same two places; not stated by the studio.

One on-screen detail is real: the mission-select screen shown in the Deep Dive is headed Welsh Campaign. It is pictured on the story page. What it does not tell you is whether other campaigns exist.