How did you get Oisten Jagerhorn for free?

By following Killing Floor 2 on its Steam store page (the Follow button), then launching the game and opening the Gear menu.

Which perk suits this character?

The guide suggests the Support loadout so you hit harder with shotguns; the character is only a cosmetic choice, though.

Lore & History

KF2: Unlock Oisten Jagerhorn Free by Following the Game

This short Hungarian guide (translated here) explains how one free playable character, the Swedish rocker Oisten Jagerhorn, was unlocked in Killing Floor 2. It was a promotion tied to following the game on Steam, so it may no longer be offered; treat the steps as a record of how it worked.

Steps

Open the Steam store page of Killing Floor 2. In the Steam overlay browser or normal store, find the Follow button under the trailer area and click it. The author's screenshot shows the button turned blue and marked as followed.
Launch Killing Floor 2 and go to the GEAR menu.
Open the character list and look for the newly unlocked character. He now appears next to the other characters.

The Follow button on the store page, shown inside the in-game Steam overlay.

The Gear menu character list: Oisten Jagerhorn sits next to Reverend Alberts, Mr. Foster and Ana Larive.

Which perk to use

The author recommends equipping the Support loadout with this character so you do more damage with shotguns. Characters are cosmetic in Killing Floor 2 and do not change perk stats, so this is a roleplay tip. For Support see the Support perk guide.

FAQ

How did you get Oisten Jagerhorn for free?: By following Killing Floor 2 on its Steam store page (the Follow button), then launching the game and opening the Gear menu.
Which perk suits this character?: The guide suggests the Support loadout so you hit harder with shotguns; the character is only a cosmetic choice, though.