The Great Healer (だいしんかん Daishinkan) can be found in many Sanctums throughout Weyard. He can usually perform one of four tasks for members of your party; revive, remove poison, remove a cursed item and drive a spirit away (i.e. heal someone who is haunted). All these tasks require a donation of coins to preform.
Serving equal gameplay roles as Great Healers are the younger-appearing Healers which seem to be distributed just as broadly and numerously as the Great Healers. It is currently unknown how one would become a healer, though it appears that they function in much the same way as a real-world church. It is also unknown how one progresses from healer to becoming a great healer (whose only noticeable difference is the clothes worn). Although, in Imil, near the Mercury Lighthouse, Mia leaves a small trainee boy behind who already possess the power to perform the same functions as a healer.
In general, healers are travelling missionaries, spreading goodwill, and are thus usually visiting the townships they reside in. Hence, they usually behave slightly differently to locals and usually make comments regarding the issues where they live. They can also provide useful hints (sometimes by using Mind Read).
The most plot-significant healer is the Great Healer in Vale, who speaks to the Wise One and tells Isaac and Garet that the Wise One has chosen them for the task of saving the world (though they can decline if they wish, resulting in a "bad ending"). Other than relaying the message, the only advice he offers is to seek the elemental Djinn, much to the Mayor of Vale's annoyance.