AKA The Vowelless, The End, the Soul Collector, or, to those who can't do without vowels, Kizke.
A powerful demon with plans to rule the world, originally summoned by Gwynn. He is very touchy about people who mispronounce his name, although the main cast invariably uses vowels when referring to him. K'Z'K's powers include the ability to raise the dead as "Deadels" under his control.
K'Z'K's natural form is that of a giant bug-humanoid hybrid, with a multitude of different limbs and an insect-like lower body turning into a (still bug-featured) humanoid torso from the waist up. This form is just large enough to hold Zoë's imaginary twin sister Pheasant in one hand and bite her head off. In diminished form, he resembles a large beetle with a disproportionately large head and a face human enough to bear expressions.
History
First appearance
Gwynn first summoned him to kill her ex-boyfriend Riff (she did not know that demons summoned with the Book of E-Ville get free run of the earth once they complete their tasks), but ended up possessed by him. K'Z'K turned Gwynn into a large purple monster and used Dr. Lorna to bait Riff into his trap. Torg managed to exorcise K'Z'K using the Book of E-Ville and grammar puns, then Riff blasted the demon into the past with Berk's time blaster.
In Medieval Europe
K'Z'K wound up in medieval Europe near the kingdoms of Mercia and Trent. He created an army of deadels to conquer both but bided his time, letting the mortals fight amongst themselves and send him more souls. He was finally moved to action when he found out that Zoë and Torg had been sent to this time from a time machine accident and were trying to use the Book of E-Ville against him. His deadels crushed the warring armies of Mercia and Trent and he captured the Book, but Torg and Zoë got away and, guided by Osric, headed for the Cave of Yffi to look for the Book of Güd. K'Z'K followed them there, but, not knowing the nature of the Book of Güd, made the mistake of looking at its pages, resulting in its power reducing him to a small, bug-like creature, which Zoë promptly stepped on.
Reapperance
After his demise, the souls of K'Z'K's victims returned to their original bodies. Gwynn's soul, however, carried a sentient part of K'Z'K's essence. K'Z'K managed to secretly amplify Gwynn's magic, so every time Gwynn used magic, K'Z'K grew stronger. He eventually appeared to her as an online lover named "Kenny" who encouraged Gwynn to use her magic more. In actuality, the "chat room" where they met was a word processor, and Gwynn was typing for both of them. When he became strong enough, K'Z'K forced Gwynn away from her friends and trapped her soul in a nightmarish dreamworld where a vortex would end her entire existence if she looked behind her. After gaining enough strength, K'Z'K took Gwynn's body back to the apartments to wreak his vengeance. When K'Z'K was on the verge of winning, Gwynn looked back, but used her friendship with Zoë to fight the vortex's pull. When K'Z'K returned to mock Gwynn, she grabbed him and threw him into the vortex instead. However, Skippy suggests that other pieces of K'Z'K exist, and another may be sentient.
Mokhadun Mythology
In the ancient city of Mokhadun, mythology tell of the existance of a creator-supergod Prozoato and a destroyer supergod Kozoaku, both created by The One. Kozoaku turns out to be the truename of K'Z'K before a spell is used to modify his truename to be more difficult to pronounce, and thus his summoning more difficult to accomplish.
As soon as Prozoato would created a spark of life in the world, Kozoaku would end it. Frustrated, Prozoato created the first god Krohnus, who wove a fate-web capable of keeping the destruction of Kozoaku at bay. To sap the power of Kozoaku, his soul was woven into the web, from where the destroyer tempts Prozoato's creations to free him.
In the epilogue, there are vague revelations that reveal that if Kozoaku's existence or fate is threatened, as one of the pillars of reality, all of existence itself is in grave danger, not just the current spark / timeline.