Little of Kai's past is known for sure, but rumor has it that his motivation to destroy the Fomors comes from the loss of a loved one.
Kai's athleticism allows him to zip around the battlefield, executing precise assaults and moving on before his enemies realize they're being attacked. His weapon can be switched between a Short or Longbow, allowing for adaption to combat situations. Or players can utilize Kai's affinity for the Cross Gun, which allows for quicker hit-and-run attack strategies.
(NOTE: As I played with Kai, it became apparent that the bow had a greater number of skills; and every one of them were best employed on bosses, or getting out of the way, whereas the Cross Gun allowed me more common croud control. The Cross Gun, I would suggest to those who have trouble keeping up with large groups of enemies.)