Kefka's appearance was designed by Yoshitaka Amano, who was given complete creative freedom in Final Fantasy VI, with only brief character outlines as guidelines. His approach was to create "real" and "alive" characters, though with consideration for the representation as small computer sprites. Kefka is well known for his clown-like apparel, which has been compared to the Joker from Batman. His dress has been described as "garish", with "makeup smeared across his face" and "a shrill, girlish laughter" that is thought to "punctuate his madness". In Dissidia: Final Fantasy, he also bore lip makeup patterned in a way that resembled a Glasgow smile.
Given only the artwork and outline to work with during production, writer Yoshinori Kitase felt that one early scene of Kefka's approach of a castle was too "boring to make completely normal" and decided to ad-lib a scene where he requests that his accompanying soldiers dust off his boots in the middle of a desert. The scene set the tone for his personality from that point onwards, suggesting that there "may be a screw or two missing from this character's head". He has been described as "powerful" and "manipulative", though he is retiring on occasion when confronted by opposition and even called a coward for delegating his authority. He is also known for his sarcastic tone and one-liners. As Kefka's power grows, his nihilism and madness grow in tandem and as he attains godlike powers he thirsts for the destruction of all life, expresses his loathing of everything in existence by chanting the word "hate" to his enemies. He is also very sadistic, taking amusement at the very idea of people dying and screaming. This is especially evident when he poisons Doma, as well as his admission prior to confronting the Returners for the final time that he enjoys destroying things precisely because precious lives are lost in the process. In his Dissidia appearance, Kefka's insanity is further explored, suggesting that his actions stem from a desire to fill his heart with destruction when love failed to provide his life with meaning. Final Fantasy VI features a song called "Kefka", which exemplifies the "sadistic joy" of the character. His laughter has been called the greatest video game laughter of all time.