Hirelings is a social feature that enables you to summon help to deal with the enemy. Hirelings can be summoned once a mission and they come with useful passive buffs. Remember though that once you have used a hireling's active ability their passive ability will disappear for the rest of your mission. The passive buffs each Hireling has is as follows:
Berserker Hirelings
Raise your health.
Trickster Hirelings
Increase the chances of your enemies fleeing a fight.
Shadowblade Hirelings
Raise your critical hit chances.