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.
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 is 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 has tricked Yuma. This is made more evident by the fact that It's 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" Witch 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.
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!.
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 Veis, 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.
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.
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!?
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...)
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 - no being forced to rely on luck of the draw here!