Mercenaries can be recruited to fight alongside you and level up as you progress. The only image released so far shows 4 mercenaries to choose from but we don't know if more are yet to be revealed. These four look like a Rogue, Paladin, Barbarian and some kind of spell caster.
The Mercenaries were designed to replicate that small-party experience. You're going to see mercenary builds that may complement certain classes that already exist in the game. So if you're running around as a Sorcerer and you need someone out there playing a little bit of a tanking role we've taken that into account with all the mercenary types you would see. - Brent Gibson, Game Director, Blizzard, 12 July 2024
Blizzard has said more about the Mercenaries will be revealed before release.
