Healers are support mercenaries that will follow other mercenaries in combat while healing them and keeping them alive. In War of Mercenaries there is only one healing mercenary at the moment, called Gentle Healer . This healer class doesn't attack it solely heals other mercenaries and the beasts. It doesn't heal other Gentle Healers though. It is still not clear if there will be more healing/ support class mercenaries added in War of Mercenaries.
The Heal Aura although it's not a mercenary is also a way to heal your mercenaries and beast for a limited time. The Heal Aura was introduced as a reward for the Rebels event. It is a very strong heal in a radius that heals everything including Gentle Healer which normally can't be healed.