Zak Monday was the anti-matter version of Zak Saturday, a malevolent doppleganger from an opposite world on the other side of the Aztec Smoke Mirror. As opposed to positive-matter Zak, he is extremely cruel and clinically insane, and is boastful about it. In his second escape to the positive universe, his only goal was to take the place of his positive matter self and use the opportunity to ruin the Saturday family. Apart from anti-matter Komodo, with whom he shares a mutual love of evil, he despises his family, and frequently betrays them or treats them like dirt.
The physical difference between positive Zak and himself is his hair color: his hair is white and black where positive Zak's is the other way around. Again in opposition to positive Zak, he held the "The Fang", the anti-matter version of "The Claw". Like Zak, he is his universes Kur, but where as Zak's powers cause his eyes and claw to glow orange, his glow green.
Being composed of anti-matter, if he and positive Zak get to close to one another, the fabric of reality begins to unravel, causing unpredictable levels of destruction. Following the Monday's first escape, he was imprisoned in the Smoke Mirror but escapes with Komodo Monday. Komodo calls Zak his servant and they go off most likely to cause trouble. He reappears in Paris is Melting, where he and Komodo Monday try and ruin the Saturday's names. They send positive Zak to the anti-matter universe to retrieve their airship, but he is intercepted by the other Mondays. Positive Zak manages to negotiate with the Mondays: they would sooner revenge themselves on anti-Zak, and so they allow him to leave in return for sending back Zak and Komodo Monday. Draining the airship of power, it no sooner takes flight, then it begins to plummet, right into the Smoke Mirror that positive Zak angles beneath it.
He appears a final time in And Your Enemies Closer, where Argost realeases him to steal his powers. Using the Flute of Gilgamesh, he extracts Zak Monday's negative Kur powers and takes his Fang, sapping him of his life force in the process. Argost uses the Fang to lead the war in War of the Cryptids.
- The Fang
- The Claw (Temporarily)
- It is Possible that "Monday" is not their true last name as Zak just called them that as "a less fun version of a Saturday." This is actually noted in Paris is Melting when he replies "Is that what you call us?"
- It is possible that because the positive matter Kur was evil, and positive Zak good, negative Kur might have been good, given that negative Zak was evil.
- The positive Zak's Kur powers are able to form a cryptid army via calming the cryptids down and bringing them under his control. Zak Monday drives cryptids insane and makes them forget what happened under his control as seen is Paris is Melting and as Argost did with his powers in War of the Cryptids.
- Positive Zak may have some concern for his negative counterpart, as he tried to warn him of Argost stealing his anti-kur powers, which would drain his life source in the process. However, he may have just been trying to prevent Argost from obtaining that kind of power.
- He resembles Albedo from ben 10 in a way.
- "No, I'm me, and you're toast!" -Zak Monday to Zak Saturday
- "They don't like us very much there." -Zak Monday
- "What are you going to do, tell your mommy? Or... Or my mommy?"-Zak Monday to Zak Saturday
- "Is that what you call us?" -Zak Monday to Zak Saturday
- "Guess who's back in town?"-Zak Monday to Zak Saturday
- "With a flute? Dude! You're crazier than I am!"-Zak Monday to Zak Saturday
- "Man, you're predictable."-Zak Monday to Zak Saturday
- "You losers don't deserve this world!"- Zak Monday to his family
- To be announced
- Black Monday
- Paris is Melting
- And Your Enemies Closer
- The Fang also appears in War of the cryptids