A priest is a support class designed for three purposes: healing, buffing, and debuffing. For this thread, we will focus on the healer build.
Why play a priest:
A priest healer excels in raids, labyrinth, and does great in arena. Having a healer is very vital in your party when you progress to the later stages.
Other players started with the priest because get a priest ally is really rare. The game does end up giving you a mediocre A grade priest, but that might not be too helpful especially when you're trying to combine for your higher grade allies. Always save your highest grade priest ally if you decide not to play a priest!
The build: This is the build I went with. It is relatively cheap and effective starting build, allowing me to stay competitive in the top 50 for arena and labyrinth. It will also safely get you to legend.
Skills:
This is a no brainer. Max out the first skill, and one point in the others. Then unlock the first skill's ultimate.
1st skill: Maxed, ultimate unlocked
2nd skill: 1 point
3rd skill: 1 point
Passives:
You would want a good amount of both the first and last passives. First passive allows you to heal back a chunk load, and the last passive allows you to do an aoe heal. I think the third passive is actually really bad for this build since you're already healing your team back to full health and the 2% defense increase isn't worth lowering your healing potential. So pretty much dump all your remaining skill points into the first and last passive.
1st passive: As many as possible
2nd passive: 1 point
3rd passive: 1 point
4th passive: As many as possible
Credits: Starter guide to Priest Healers by 2BVicious