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.
Getting your Mercenary
No information has been officially released on gaining access to mercenaries. Diablo 2 and Diablo 3 both had Mercenaries for hire with their function being improved between those games. It's expected that Diablo 4's Mercenaries will be even more customizable and therefore play a more useful role.