The game

Is Kingmakers single-player?

Yes. The campaign is built to be played alone and works without a connection, and up to 4 players can also run it together. The developers describe it as a single-player campaign that is also playable in co-op, not the other way round. What alone does not mean is fighting alone: you still command an army, and later missions are not meant to be soloed.

Yes

Solo, offline, with a tutorial and no forced matchmaking, by the developers’ own answers. What would change this: a developer post limiting solo play, or the store pages dropping the Single-player label.

A helmeted rider on a motorbike carrying the number 11 crosses a field of tall dry wheat, with pine trees and a domed building on a distant hill.
A lone rider on a numbered motorbike crossing a field of tall wheat. Frame at 3:54 in the Gameplay Deep Dive, tinyBuildGAMES.

What the developers say

FAQ

Solo, offline

Yes, you will not require an online connection to play solo.

Official FAQ. From the Kingmakers Official FAQ (Steam discussions)

Deep Dive

Designed for one

We’ve designed Kingmakers to be a great single-player experience.

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

Studio account

Never changed

An account carrying the developer badge wrote that the design has not changed since the first announcement, and that It has always been a singleplayer campaign that’s also playable in coop.

How solo play works, as far as it has been said

Said: The campaign can be played soloThe official FAQ says the campaign mode has a story you can play solo or co-op.
Said: Solo needs no connectionThe FAQ answer is a plain yes. It was written before the delay; the later replies about solo play do not touch it.
Said: You are not forced to match with strangersThe game director says private lobbies already exist in the current builds and that the studio is not going to force anyone to play with others.
Said: A tutorial is built inAccording to the director it is built into the first mission, so a solo player is not left to guess.
Open: A sandbox modeA publisher reply from before the delay says there will be one alongside missions. It has not been shown; see the Kingmakers campaign map.
Open: Taking a solo save into co-opNothing has been said about it.

The private-lobby and tutorial answers are quoted in full on Kingmakers co-op and multiplayer and the Kingmakers Q&A. The director’s wording on lobbies was that You can play in singleplayer. You can have a private lobby and invite just your friends.

A helmeted soldier seen from behind fires a rifle down a red-carpeted stone hall lined with statues and banners, with flames at the muzzle and bodies on the floor.
A soldier fires down a red-carpeted stone hall lined with statues and banners. Frame at 4:18 in the Gameplay Deep Dive, tinyBuildGAMES.

What alone does not mean

Playing solo removes other players, not the enemy armies. The Deep Dive puts it plainly: Even with an army at your side, you’re still heavily outnumbered. You recruit and command that army from the strategy view, covered on Kingmakers units and officers and Kingmakers base building.

The director is more specific about how far one player can get without it:

Paul Fisch Game director, replying on the Steam forum

A man by himself with an assault rifle loses against 30 archers.

Maybe not at the very start, but on harder difficulties and later missions, you really can’t solo everything even with air support and turrets and landmines and drones.

Thread: Paul Fisch on armored enemies and difficulty, Steam discussions

So a solo campaign is a commander’s campaign. The difficulty side of that is on how hard Kingmakers is, and how a co-op group divides the same battle, each player with an army of their own, is on Kingmakers co-op and multiplayer.

Where the store pages and the shows stand

Steam files the game under Single-player alongside Multi-player, Co-op and Online Co-op, and the Epic Games Store lists Co-op and Single Player as its features; the full list is on Kingmakers crossplay. The one build the public has played, at BiliBili World, was single-player, as written up on the Kingmakers BiliBili World demo, while the planned demo is described as fully multiplayer; see the Kingmakers demo. What the first Early Access release includes is on what Kingmakers Early Access includes.