"Our army's medic. Keep our troops healthy so they can wreak more damage."
The Healer heals injured allies on the field of battle. Her healing ability effects all units in a rather large radius. She will seek out the closest injured allies to heal, and won't stop healing them until they are completely healed. This can be dangerous if a high HP unit such as a Fire Demon or Oni Troll goes off by himself, leaving your main army without a healer.
Once the unit she is healing is at full health, she will move to heal a new target, even flying over walls to get there.
Like the Essence Master, the healer is extra vulnerable to Lightning Tower, which will seek her out and zap her for +300% extra damage. Luckily, her heal radius is just outside of the Lightning Tower's chain attack range, so typically she is safe from the tower unless there are units close enough to bridge the gap.