The caretaker of the Duel Lodge and a personal friend of Yuma's grandmother, he lives all alone in the mountains with his pupil, Yamikawa. How he manages to survive so long is anyone's guess. Grandma Haru periodically sends supplies to him via her grandson, Yuma. He uses a lot of Yu-Gi-Oh! era cards such as the Poker Knights archetype as well as the classic Mirror Force.
The Demonic door that only appears in Yuma's dreams. It warns Yuma of future dangers and gives him new powers in order to fight them, but all this help has a price to pay...
A physical manifestation of the card with the same name. Tries to get Astral to become evil as he claims that Astral wants the same thing all numbers want: destruction.
As of episode 101, he became the owner of Numner 65: Djinn Buster. In the following episode, he upgraded it into Number C65: King Overfiend, and he himself was Ranked-Up into Number C96: Dark Storm.
In episode 110, he swiped Number 69: Heraldry Crest and Number 92: Heart-eartH Dragon. And then ranked them up into Number C69: Heraldry Crest of Horror, and Number C92: Heart-eartH Chaos Dragon.
He was finally defeated in episode 111 but this came with a high price: In a last ditch attempt to take over Astral's body Black Mist turned himself into a spear-shaped object and stabbed Astral. Astral used his remaining strength to destroy Black Mist but this forced Astral to return to the Astral World as a result.
Actually a fragment of power from Don Thousand, which was why he was different from the other Numbers.
- A God Am I: Goes mad with power in episode 110 and declares himself to be a god.
- All Your Powers Combined: Can absorb the powers of the other number cards to make himself more powerful.
- The Assimilator: His field spell, Chaos Field, allows him to take Yuma's Numbers and Rank them up into Chaos Numbers at will.
- Ax-Crazy: Slips into this territory in episode 110.
- Badass Boast: It originally claimed to be "the most powerful Number". And it is, just not from a card effect angle. It has more direct control over things than any other Number though.
- Big Bad Duumvirate: Temporarily with Vector.
- Big Bad Wannabe: Especially after his return. He has a phenomenal amount of power, but no real vision or allies, and loses every onscreen duel.
- Corrupt Politician: Not technically, but after escaping from the Emperor's Key up to the point where it allies himself with Vector, he uses such a man as a host; whether the Congressman was corrupt even without its evil influence isn't known.
- Dark Is Evil: He is dark in color, has dark powers, his card self is a DARK Fiend-type monster, and he is out to destroy everything just because.
- Demonic Possession: He takes over Astral's body in his first appearance. He retains this form every time afterwards. Also, after Astral undid the seal on him in Zexal II, he escaped to the human world and uses a human host.
- Enemy Mine: Briefly calls a truce with Yuma and Astral to fight the Shadow Giant, a demonic creature sent by the Door to test Yuma.
- Evil Counterpart: Becomes one to Astral. They even have the same VA in both the Japanese and English versions.
- Expy: He's primarily one to Marik - consistently a villain, more the nemesis of Astral than Yuma, holds important information about Astral's memories but isn't willing to give it up, and gets much stronger and much crazier at a certain point. He even has a Villain Team-Up with Vector, the show's Bakura expy.
- Eyes Do Not Belong There: In chaos form, he has a big red eye on his abdomen. Also, a common trait with his ace card, No. 96: Dark Mist and Dark Storm. The former has eyes around his shoulders. The latter has eyes on his hind legs.
- Hannibal Lecture: Pulls one in his first duel with Astral, telling him to abandon Yuma and fight back, trying to convince him that Yuma betrayed him anyway, and the reason he can't absorb Chaos Numbers is because he needs the power that he has. While Astral doesn't believe a word of this, he pretends to do so for a few minutes to make Black Mist thinks he's torn.
- I Surrender, Suckers: This was the most vital part of his first plan (he forced his original holder to lose on purpose to trick Astral into absorbing his card, then waited until Yuma was separated from the Emperor's Key so he could strike.
- Improbable Weapon User: His "Malicevorous" cards in Zexal II wield giant kitchen utensils.
- Not Quite Dead: As shown in episode 112, Dark Mist wasn't destroyed or absorbed by Astral but rather he was sealed in a sphere of his own power which is in turn absorbed by Vector and Don Thousand.
- Omnicidal Maniac: He has always wanted destruction, and it's all he seems to think Astral wants as well. Becomes even more of one in episode 110 - he wants to lay waste to all three worlds.
- One-Winged Angel: When he ranks up his card self with Barian's Force, he turns into a tall hulking Winged Humanoid with an eye in his abdomen, gaining quite a resemblance to Don Thousand.
- Obviously Evil: Dark? Demonic? Wants to destroy everything? Yep, he's as evil as he looks.
- Offscreen Moment of Awesome: He defeated the Battle Ox who guarded Number 65: Djinn Buster in a duel that occurred offscreen.
- Palette Swap: Appears as a pitch black version of Astral.
- Red and Black and Evil All Over: Initially, he was just black in color with a few red and green lines here and there, though he was self-serving enough to want Astral's body, take the numbers for personal power, and want to destroy everything. He gains a lot of red on his body in addition to his normal darkness-based color while remaining evil.
- Sadistic Choice: In his first duel with Astral, he pulls an even better one than Vector. The temple places Yuma in a Death Trap so that a bladed pendulum swings closer to him every time Astral inflicts damage upon Black Mist (in other words, Astral will kill Yuma if he wins, and losing would cause his own death, as the duel is over their Numbers). Fortunately, a third option does exist, but Astral has to get Yuma to figure out how to get to safety by himself, which he does.
- Slasher Smile: Likes to flash it quite often◊.
- Taking You with Me: Non-lethal example in his first duel with Astral; rather than lose the duel, he evolves into his Chaos form and uses a Trap that reduces both their LP to zero at once, forcing a draw. As such, Astral isn't killed, but neither is he.
- Tentacle Rope: Uses his tentacles to tie up both Yuma and Astral on separate occasions.
- Villain Has a Point: He turns out to have been absolutely right in his claims that Astral was created to destroy.
- Villainous Breakdown: In episode 111, he goes ballistic when he realizes he's going to lose to Yuma and Astral.
- Villainous Rescue: In episode 89, when Astral and Yuma were in a tight spot and they needed his card to stretch out a victory, he aided them in exchange for being set free from his imprisonment.
- Villain Team-Up: With Vector. He teams up with the Barian and gains "Rank-Up-Magic Barian's Force" in order to get Number 65: Djinn Buster and have a shot of defeating Astral and Yuma.
- Winged Humanoid: In chaos form, he sports a pair of thick, demonic wings growing from his back.
- Would Hurt a Child: Tried to impale Bronk with tendrils to gain back Number 39: Utopia. Bronk is only about 14 or 15 years old.