Mercenaries in Diablo 4 were introduced with the Vessel of Hatred expansion. These are far more than just simple hired guns. They also contribute to the world’s lore and interact with players, acting as companions and offering both combat support and added gameplay mechanics. The four distinct Mercenaries each have their own personalities, skill trees, and playstyles. It also adds a new dimension to the loot grind, with the Rapport system rewarding players with powerful rewards for their dedication to their chosen Mercenaries.

https://screenrant.com/diablo-4-vessel-of-hatred-all-mercenaries-which-to-unlock-first/