Jack Noir is an antagonist from the webcomic Homestuck. In this comic, the characters play a high-stakes "game" called Sburb that involves reality alteration, and the creation of worlds and universes. In the game, there are two competing factions on the worlds of Prospit (white) and Derse (black). Jack Noir is the Archagent of Derse, a high-level bureaucrat reporting directly to the Black Queen.
The war between Prospit and Derse is influenced by the actions of the players. The players are guided in the game by a Kernelsprite. This sprite takes the form of whatever objects the player "prototypes" by placing them in the kernel. The agents of Prospit and Derse also receive the effects of this prototyping, changing their appearances accordingly. The first two players, John Egbert and Rose Lalonde, prototyped a one-armed harlequin doll, a princess doll with tentacles and a dead cat between them, forcing the agents of Derse and Prospit to wear silly costumes. The third player, Dave Strider, prototyped a dead crow impaled on a sword.
Each group of players spawns a new session of Sburb, and the game has a "hyper-flexible" mythology that tailors itself to that particular group. One of the few constants is Jack Noir and his contempt for his superiors, the Black Queen in particular. He spends most of his time in his office avoiding paperwork and handing out sword and hit lists to anyone who wants a favor from him. He also has detailed plans for usurping and exiling the Black Queen, providing savvy players the opportunity to team up with him and avoid fighting one of the most powerful enemies in the game.
Jack Noir had no love for his princess clown costume and refused to wear it. The Black Queen, ruler of Derse, insisted he did and indeed seemed to enjoy his humiliation. By chance, he had acquired a box containing a very powerful weapon lost by the players, which he used in a fit of rage to kill the Black Queen and take her ring. The ring granted him a massive power boost and transformations based on the prototypings: He lost an arm but gained two tentacles at the waist. His head morphed into a mangled jester's hat, he grew wings and cat ears and a sword appeared lodged in his chest (which he could remove without injury).
Having taken the effects of three prototypings (the fourth had not yet progressed that far), he took control of Derse and became known as the Sovereign Slayer. He flew down to the battlefield and began slaughtering Dersite and Prospitian alike, destroying the scepters of the Black King (which hold the same power as the Queens' rings). Using the White King's scepter, he brought down a rain of meteors onto the battlefield, which transported them to Earth. He then proceeded to demolish Prospit.
When the time came for the fourth player, Jade Harley, to prototype her Kernelsprite, her dog Becquerel jumped into the kernel on his own. Becquerel is known for having ridiculous, if vaguely defined, powers over space. In his final transformation, Becquerel's traits seemed to overwhelm many of the other prototyping elements; he lost his tentacles (although he can still extend them if desired) and jester's hat, and his head took on a canine appearance. In addition to the standard power boost the fourth prototyping would have given him anyway, he also gained Becquerel's absurd reflexes and powers of teleportation and super speed, and is usually seen surrounded by a telltale green and yellow shifting energy field.
While no more powerful than a king or queen would be in his place, Jack's disregard for the rules and established order of the game make him incredibly dangerous and unpredictable. He is currently, as Dave put it, "In charge of random teleportation murders," having made a habit of appearing suddenly and killing people in an attempt to find an opponent that can challenge him. This search has so far proved fruitless; he has killed even the most powerful opponents, like Bro and Rose, with very little effort.
However, Jack has two weaknesses. The first is Becquerel's love for Jade. When he was prototyped, Jack found himself unable to harm Jade, or even allow her to come to harm. The second is his attraction to the Prospit Exile Parcel Mistress, who, upon putting on the White Queen's ring, now has the exact same powers and abilities as Jack. She hates him with a burning passion and is currently hunting him down in revenge for killing her beloved, the Wayward Vagabond.
As Jack is chased down by the Parcel Mistress, he gets mad at her and decides to fight back. As they battle, Jade interrupts and ends up getting punched in the face by the Parcel Mistress. Jack gets distracted by this and the Parcel Mistress cuts his arm off and steals his ring before punching him in the face, which sends him flying straight to the ground. After the battle he is patched up by Ms. Paint whom he seemingly attaches to. Since he isn't killed on-screen, it's possible that he redeemed himself to sociolize with help of Ms. Paint.