Unlike many other card games, using an equip card in Duel Monsters involves assigning it to a target from a different card slot, not stacking them on top of one another. Thus, the first time that all three axis of the game board have been relevant is in the overlay mechanic of the XYZ monsters! Literally a new dimension of play!
The reason Yuma sucks so much at duels is because his family forbade him from dueling. He's in effect a complete beginner who has had to figure out the rules as he goes along.
Shouldn't make him completely unwilling to accept help though, and he's even worse than your basic duelist who had no training. Even they would have heard the rules.
Plus, for someone who was "forbidden to duel", Yuma demonstrated no difficulty in repeatedly dueling without consequence from his sister, the one who'd "forbade" him in the first place.
The reason Yuma has Number 39 is because the number is a pun! 3 is San and 9 is Kyu, San Kyu means Thank You!
The reason Number 55 is a Gogogo Monster is also because of a pun. 5 is Go, so taking 55 as 5 and 5 makes it translate to Go Go. So essentially the card's full name is Number Gogo Gogogo Goliath.
When Yuma and Astral pull off a Zexal Morph, they get surrounded by auras, Yuma's being red and Astral's being blue. This aligns with their personality types, with Yuma being the Red Oni and Astral the Blue Oni.
Amusingly, this means that Yuma has a FIRE attribute and Astral has a WATER attribute.
The fact that Yuma is able to perform two shining draws in his duel against Vetrix despite only ever using one in a duel before makes sense, considering that Yuma would have two overlay units as a result of his XYZ change.
ZEXAL weapons only being equippable to Hope Ray, a Chaos Number, doesn't make much sense considering it's supposed to be a legendary power from the Astral world that can only be played on a card that requires the emotions of a human. Then comes the Barians, who all seem to have Chaos Numbers, and it suddenly makes more sense: Astral could likely have converted Numbers to Chaos Numbers by himself, but just forgot how.
The Numbers have been stated to be based on, or are symbolic of, the person who first used them. Now let's have a look at Vector's personal Number "No. 104: Masquerade Magician - Shining" why would Vector have a Number with "Shining" in the name? It's been revealed that Rei Shingetsu is Vector. Meaning that he's tricked Yuma. This is made more evident by the fact that its Chaos Number counterpart is DARK as opposed to this monster's LIGHT.
While we're on the subject of Vector/Rei. During a tag duel he let Yuma have "Rank-Up-Magic: Limited Barian's Force" which Yuma used to summon Utopia/Hope Ray V. Notice anysimilarities between that monster and Vector? Hope Ray V is Foreshadowing that Rei is Vector.
Further evidence: Rei's Shining cards are all relatively inoffensive-looking monsters, with a darker, twisted version copy hiding until it's effect activates or it attacks. Just like Rei is a relatively inoffensive-looking persona over the dark Vector.
Some laugh at the fact that Vector did a Dandylion cosplay, but look at its effect: It allows you to Summon two Tokens when it goes to the Graveyard. Possibly a hint at Vector's ability to clone himself.
The English dub takes the foreshadowing a step further with the name "Ray Shadows". The last name is much more obvious, but in optics and mathematics, a "vector" and a "ray" have similar meanings.
When "Rei" tells Yuma that Shingetsu "is an act the world must buy", he's not just talking about people thinking that "Rei" is a normal kid. He could also mean that "Rei" the Barian Guardian is an act that Yuma has to buy (which he does).
Now that we know for sure Shark and Rio are Barians, I was watching the second ending of the show "Longing Freesia", and noticed that at about 1:15, there's a scene that involves Shark summoning Black Ray Lancer and attacking Kotori with it, before Yuma uses Utopia to protect her. This pretty much foreshadows that Shark was a Barian this whole time, specifically Nasch, the leader of the Barian Emperors!.
This also explains why Tron wanted to use Shark as his assassin against Dr. Faker. Since he had come into contact with the Barian World, he knew that Shark was connected to it. So, what better way to get his vengeance than by using someone who comes from the place he got his power from.
Shark being a Barian suddenly makes a lot of sense, when you look at his previous ace, Shark Drake. He was capable of Chaos Overlaying it for Shark Drake Veiss, which could also be construed as a form of Rank-Up. Only people with powers or influence from the Barian, or Astral, have been shown to be able to Rank-Up.
At first it seemed a bit random that Shark dueled Durbe while Yuma dueledVector and Kaito dueledMizael in Sargasso the Different Dimension Ancient Battlefield. Until you realize Shark is Nasch and that Durbe and Nasch were incredibly close friends in their past lives. Very close friends who ended up fighting each other.
It also explains why when Durbe saved Shark while in the first Number ruins, his Baria Lapis was repaired. Shark, as Nasch, has Barian powers and unknowingly used them.
Why do the true forms of the Fearsome Four of the Dark Dueling World look like monsters instead of more proper Energy Beings? It's the fact that the Numbers they use are fake. Unlike the over hundred Numbers that are connected to the Emperors, along with the Numbers that represent the Seven Emperors past lives, which are the real deal.
The Numeron Code has a basis in previous series; Darkness said before that the universe came from a single card, with the World of Darkness being the back side. As the Yu-Gi-Oh! Wiki points out, the front side would be the Numeron Dragon/Numeron Code.
It's no wonder Yuma was having so much trouble dueling when he tried leaving Astral behind at the start of Zexal 2. As we see in the next episode, he'd already reconstructed his deck to include large amounts of Number support (Let's say Faker designed them as a gift), which would usually require Astral's presence to be useful. If 39 hadn't re-materialized on it's own, all those cards would have just been clogging up his deck.
In the final duel, Astral uses Number 25: Force Focus, a Number Card completely unrelated to Yuma, Shark, or Kite. However, in the episode it was introduced, it gave the holder the ability to take pictures of a potential future. This was another way for Astral to remind Yuma that he has a future waiting for him that had yet to be decided.
It also was the last Number used by someone that wasn't actively hunting them, so its inclusion in the last duel is fitting.
Each of the Chaos 100+ monsters has some way to counter their respective Legendary Number, symbolizing Don Thousand's desire to keep their holders' true memories sealed away. The only exceptions are ironically 101 and 103, the numbers of Kamishiro siblings, who counter each other's respective Legendary Number.
C101's life gaining effect can completely nullify the battle damage dealt by Number 94 and its attack halving effect before absorbing Number 94 with its other effect the next turn.
C102 can negate its own destruction by card effect, thus preventing the Sadistic Choice of Number 44's effect and even damaging its opponent for trying to do so.
C103 can use Number 73's attack doubling effect against it by burning 73's wielder when 73's attack changes.
C104 has an effect on-summon that will trigger 65's effect, which C104 itself can then negate with its other effect, sending a card from their opponent's hand to the graveyard and halving their life points.
C105 can destroy a monster with its effect, bypassing Number 54's battle protection and damage reflecting effects. For an added bonus, destroying 54 with this effect will also inflict 100 damage to the opponent, the same amount of life points they would have if they'd previously used 54's other effect to save themselves from losing.
C106 negates the effects of Number 64, allowing it to be attacked instead of its token.
C107 can't use any of its effects while Number 46 is on the field due to being a Dragon-type, but its attack points are high enough to destroy 46 by battle anyway.
Given what Shark turns out to be (namely the leader of the seven Barian Emperors), it's quite appropriate that the first Number he gets, in the very first episode no less, is Number 17.
At first, many people might think that Dr. Faker's name is due to him tricking Kite into collecting Numbers. But then it turns out the guy was just a Well-Intentioned Extremist. What was fake was his Obviously Evil appearance and persona.
Unsure if this is brilliance or horror, but the dub playing "Battle of The Dragons", usually reserved for Number Monsters being summoned when New Order 13: Etheric Amon is being summoned. Eliphas' desire for order above all else is just as corrupting as Numbers are to hosts without strong wills.
Shortly before the Legendary Numbers arc, Shark and Rio have a duel, where both players use effects that let then use Xyz monsters as Xyz materials. This is a more roundabout way to to Xyz Evolve, foreshadowing their eventual reveal as Barians.
Remember those rockets Anna shot at Yuma in her premiere episode? Let's just say that if Yuma didn't have quick reflexes, this series would have had a very messy end.
Number 30: Acid Golem effect makes it that, unless you give up one overlay unit, that the controller takes 2000 points of damage. Number monsters take a form based on whoever they belong to, okay, now whose card was that in the first place!?
This troper has thought about this as well, but instead of thinking about the guy who made Acid Golem, I thought about the guy who made Black Mist.
Mist was originally just a manipulative blob. It didn't turn into an Eldrich Abomination until Yuma got it...
Episode 112 reveal that Black Mist is a shard of Don Thousand's malice that somehow ended up trapped inside Astral during their last battle.
Manga entry: Yuma and his friends had no problem eating what was left of Captain Corn after he was killed. Think about that for a few minutes. (Corn or not, eating a sentient being is a little...)
Kaito's (temporary) death. We don't see his corpse, but that might be for the best, considering that death by exposure to vacuum might very well be one of the most brutal franchise deaths to date. If this wasn't Yu-Gi-Oh, we might have thought he had been Killed Off for Real!
In the dub of Episode 144, Yuma all but outright says that he failed to save Shark, Rio and Kite, meaning they are all most likely dead for good.
Nope in the last episode everyone is shown to have been resurrected via the Numeron Code.
Yuma adds Numbers cards to his deck when he defeats a Numbers user. Numbers, being XYZ monsters, go in the "Extra Deck". The Extra Deck is limited to 15 cards. Yuma will have to pick and choose which ones go in.
It's not like he will summon anything other than Utopia, though.
Don't all Number cards disappear when he isn't dueling? I remember it being a point in one episode.
Firstly, the Extra Deck limit doesn't seem to exist in the anime, it's never stopped Yusei and given that it never applied before him they probably decided not to change the rules. Secondly, Yuma does summon Xyz monsters other than Utopia, although it's usually when Astral is the dueller that other Numbers are used. And Finally, the Number cards (eventually Utopia being the exception... but not early on) don't appear unless Astral is taking part in the duel as they are IN Astral.
I would say that Astral holds onto the Numbers, and Yuma's true Extra Deck only has his non-Number Xyzs (like the Djinns, Excalibur, One-Eyed Skill Gainer, and the like) and that Astral supplies him with Numbers as the need arises. I'd also like to point out that while Utopia is his favorite Number, he has used others on occasion, even when he wasn't required to, like when he used Terra Byte in the duel against Daisuke. (You know, the guy who used the Coach King guy...)
This can easily be explained with a rudimentary understanding of philosophy and advanced physics. Yuma's Extra Deck is probably working with the Anthropic Principle, which states that the universe is only certain when observed by intelligent beings. When the Extra Deck Box is closed, the Extra Deck can't be observed so its contents are in flux. The non-Number cards are always present, but space is left available for Number Cards. When the box is open when Astral is present, a collection of Number Cards are present that are in line with what Yuma wants to summon. When Astral is not present, there are no Number Cards. Until the Number Cards are put into play, they are not officially part of the deck, so Yuma can bring out as many different Numbers as he wants until he reaches the size limit of the Extra Deck. When that limit is reached, Astral will stop supplying new Number Cards.
Astral World is established as a world of complete order, but things go bad when they try to expel ALL "chaos" and thus create Barian World. In Yuma's duel with Eliphas, it's established that Eliphas can "Shining Draw" as much as he wants. As Yuma himself might point out: What fun is a game when you ALWAYS pull the card you want and ALWAYS win?
The implication is that normally both sides can Shining Draw, evening the playing field by having both sides able to get what they need. The reason Yuma vs. Eliphas is more uneven is because as Eliphas states, Yuma isn't an Astral World being and doesn't have access to Shining Draw on his own.
There's also the fact that Eliphas isn't the kind of person who would care about fun.
Also, Shining Draw and being able to control exactly what you have in your hand sounds like a pretty in-character way for an entity that is essentially order incarnate to duel - not being forced to rely on luck of the draw here!
Kaito and Haruto's mother likely died in childbirth or shortly thereafter. Apart from having no appearance or even mention, (flashbacks included), it would explain why Kaito and Faker both go to such lengths to protect Haruto; with his sickly nature, they're doubly afraid of losing him since they have already experienced loss, which is why they are willing to cross the Moral Event Horizon. Considering Faker's numerous drastic actions, one would assume a mother would have at least some involvement, unless she wasn't there at all. The most obvious instance is when Kaito attempts to flee Heartland City with Haruto, knowing that Faker was up to no good and realizing that it was not a suitable environment for a child. Additionally, it is revealed that Kaito originally constructed Orbital 7 as a nanny for Haruto, to fill in the physical roles a mother would perform for a child. Furthermore, Haruto's frailty could be explained by a premature birth that his mother's body could not handle, or that it was known that she was dying and a Cesarian section was necessary to save his life.
(not counting the other Signer Dragons or Shooting Quasar in the final duel, as we clearly see him swap out to use them before going to the duel.)