Kappa was an advisor to Prince Varson of Athanor while he was claiming the crown from the rightful king, King Trion.
In the second book, Alex tricks Kappa into insulting him. He then proceeds to turn Kappa into a donkey, leaving him with no influence over Varson. When this happens, Alex is able to convince Varson to blow the Horn of Moran, thus proving that he is not the true king of Athanor.
After Varson's plot is defeated, Kappa is punished by remaining in the form of a donkey.