I dropped this anime at first sigh, I only watched 3 episodes and I knew it wouldn't have a good premise. But many years after this lame attempt, I tried to watch this until the end and accomplished that mission. Plot: The MC is Tsukumo Yuma, a 13 teenager boy whose motto is "Katobing" that means "I'm going to rock" or something along that line (I watched it in my native language so I don't know how I should translate this). He made a pact with the Emperor's Key, the necklace he uses along the series, to obtain a new power. So he is now able to see a duel spirit named "Astral", and they are bounded by that "contract". By now, Yuma is the only one able to see Astral so other characters think he is speaking alone. Astral reveals that he came from another world (yea, isekai boyz) but he lost his memories and the only way for him to recover them is by colleting the "numbers", exceed (XYZ in english translation) cards that spread around Heartland City (the place where the plot revolves). So Yuma have to help Astral fullfill this mission since they are bounded and it means if Yuma lose a duel, it means Astral will disappear and at least Yuma is empathic to help him. Basically, when Yuma duels, the more his HP is close do zero, less visible Astral becomes because he will, as I said, disappear if Yuma is beaten.
First thing is that compared of all of the MCs from the previous anime franchise, I found this boy (Yuma) the most irritating of all. He is childish, impulsive, acts before thinking and all the characteristics you would find... annoying, yea. He lacks tacts and dueling skills, that later will be compensated because he starts do listen to his partner, Astral, who is good at duels. At first they don't get well, but after a couple of episodes, they start to get along and Yuma starts to think of Astral as his friend.
Yuma has a lot of friends he can count on: Kotori, Tetsuo, Catty-chan, Tokunosuke and the class president who I forgot the name. And it seems this helpless boy has 2 girls crushing on him (don't know wtf they see on him but love is love)... Later, he and his first rival and also the first antagonist in the anime, Shark, become friends because Yuma is really persistent. He thinks that he is friends with all people whom he dueled and that by dueling the duelists will know more about each other (the same thing after 3 franchises repeating over and over again). From his friends, Shark and Tokunosuke were possessed by the "numbers" and Yuma defeated them and Astral was able to retrieve pieces of his memories. And yea, the "numbers" possess their owners and reveal all the darkness in their heart (this one reminded me a bit of the first anime).
The anime progress by Yuma collecting "numbers" and a lot of eventful things happen along the course. The most memorable was the Duel Carnival Festival promoted by the most powerful and influential man in the city, Mr. Heartland. In this event, duelist from all around the world will duel and have to collect six pieces of heart parts to make a full heart. They collect them by beating duelists. Some people will stay in Yuma's way, a shounen classic tactics. In this arc, some new antagonists are revealed, as well as an anti-hero, Kaito, who Yuma tried hard to befriend, but Kaito reject a lot of his attempts. From that point on, the plot seems to become 'okay', there is some twists here and there and the past of these new characters are revealed, to the point of feeling empathic towards them. Yuma's father past is also revealed too, he is the one responsible for lefting this weird "Katobing" motto Yuma uses all along the series.
The end were left with lots of questions without answers and it frustrated me. Personally, I only watched this because I wanted to know about a specific character backstory and for this point, it really surprised me. I wouldn't recommend this one to anyone, you can watch Yu-gi-oh classic, Yu-gi-oh GX or 5D's, they are a lot superior compared to this one but feel free to make your decision.
About the other points, I found the animation okay, wasn't the best but wasn't the worst. I watched it in mid quality because I was not able to watch in HD, if it has it's option. The sound were good, there were a lot of unique speechs traits in the characters, some were even funny (I watched this in original dub - japanese) and the effects sounds were also good as well. To be sincere, I did not pay attention in the OPs and EDs to opine.
The characters were overall okay, some were well developed some not. Yuma had a good development even though I still find him annoying, but he became less annoying as the show was about to end. Personally, I liked Shark, Kaito, Four and Three the most. They really had an interesting but sad backstory. That's all of it.