|Name||Kaos, Ronnie Soak|
|Occupation||Left the Horsemen before they were famous, now is a dairyman|
|Physical appearance||Completely black eyes|
|Books||Thief of Time|
|Cameos||The Celebrated Discworld Almanak|
Kaos was the fifth horseman of the apocralyptic riders, but he left them before they were famous because of artistic disagreements. Of course, he always said later on that he never cared about their success. He is an anthropomorphic personification. Yes, Kaos is written correctly. Kaos is the real chaos. It is chaos with the complete absence of rules and not the chaos which can be used to draw pretty Mandelbrot patterns. The ancient Kaos would not believe in all seemingly messy things obeying and indeed revealing an underlying universal rule.
Many anthropomorphic personifications dislike the Auditors of Reality because the Auditors like to break actual phenomena in the world in order for things to go according to what they think are the correct rules. Kaos hates the Auditors for the simple reason that they embody rules.
Kaos has eyes that are completely black from edge to edge. He wears a helmet with full-face visor, on which there are strange black spots that might look like the wings of a strange butterfly, and then again might look like the eyes of some alien creature. He rides a chariot with a black horse which glows slightly red. His sword has a blue flame, burning with absolute coldness. His hands trail with cold steam.
Since, as mentioned above, Kaos was the fifth horseman of the apocralyptic riders, and that he'd left them before they became famous, it would seem that these 'disagreements' were primarly between War, Famine and Pestilence, against Kaos. Kaos, for the most part, it seems, gets along rather well-enough with Death, (although Kaos thinks of Death as "a Big Sleep.") According to a discussion between Kaos and Lu-Tze (one of Kaos' 'creatures'), Kaos was actually the first of the apocralyptic riders to be such, before the other four.
Currently he delivers dairy products in Ankh-Morpork under the name of Ronnie Soak. (Please, read his last name backwards.) Since physical laws such as time are optional for him, he is always on time and can attend to a large number of customers each day (exactly 7 o'clock, not a second late, at every customer's door). Ronnie also gets to go and get the best milk from everywhere, including extinct species and from non-mammals. When not acting as an apocralyptic horseman, he uses his sword to keep the milk, butter, yogurt, cheese, eggs, etc. nice and cold.
After a talk with (or, more correctly, after being talked at by) the famous History Monk, Lu-Tze, Kaos decides to accept the role of Chaos as theorized by modern people: seemingly completely random events all obeying a universal rule. In that position, Chaos trumps everything. Ancient disorderly Kaos had been pushed aside by objects, life forms, and societies, but Chaos is the principle of absolutely everything.
Kaos also has a lot in common with The Lady; case in point, the Butterfly effect. The butterfly motif on Kaos' helmet is reflective of the Butterfly effect, in which a small change can have any number of effects on subsequent events. Both Kaos and The Lady have exhibited the use of Probability in their approach to what they each do. They have even been connected with the imagery of this 'very special butterfly', also known as the 'Butterfly effect': In Interesting Times, The Lady, when beginning a new game against Fate, released a small butterfly from Her hand.
What swings it for Ronnie and clinches his decision to rejoin the band for this gig is a seemingly artless reference to the Auditors as "the Laws."
This is another (at least)bi-level reference. On Roundworld, the existential bandits Bonnie and Clyde, and John Dillinger, were admittedly motivated by getting rich quick: but they were at least as drawn to crime by a distaste for "The Laws" (a Texan abbreviation for agents of law-enforcement). Dillinger, a free-spirited and raffish bank-robber, saw hitting out at The Laws as his method of throwing grit into the machine.
Use of the term "Law" in this context also evokes Michael Moorcock's cosmological view: that the primal struggle in the world is not between Good and Evil, but between the opposed and co-equal principles of Law and Chaos. (Incidentally, Moorcock's revision of the tired old "Good Versus Evil" rationale in fantasy fiction also serves as the jumping-off point for role-playing fantasy games such as Dungeons and Dragons, where players roll dice for "character alignment". A veteran DM such as Terry Pratchett would know this, of course...)
Naturally Chaos would be drawn to fighting Law... the song-lyric might be re-written as I fought the Law and Chaos won.
The fifth horseman who left before they became famous is an obvious reference to Pete Best, the Beatle who was sacked before they became famous.
The butterfly motif on Kaos' helmet is reflective of the Butterfly effect, in which a small change can have any number of effects on subsequent events.