I have been watching the three diff series and have seen that the characters[not the villains] are similar in terms of their outfits, attitude & more. Well...here's some of them that I know.
The main characters - hmm they wear jackets and has a fierce/semi-fierce looking eyes that makes them look cool hehe [atemu in this case, not yugi and 3yr Jaden]
Joey, Crow, Syrus & Tyranno - best/loyal friends of the main character [Joey and Crow are the funniest 
]
Kaiba & Atlas - Rivals and have "no one can beat me" and "self-claim the most powerful duelist" attitude. They also wears a white "dress-look" outfit that flutters everytime they walk. Well, Atlas at least stick with Yusei's gang all the way in the journey but not kaiba.
Atemu, Yusei, Kaiba, Aki - occasionally smiles or laughs [they are too serious..hmm.. is there another reason for this? ]
Tea, Tristan, Duke, Rua, Syrus & other sidelines [too many to list] - the cheerleaders and of course they duel sometimes too
Yugi/Atemu, Ruka & Jaden - able to see the duel spirits and have a fuzzy duel spirit each hehe . [Kuriboh, Winged Kuriboh( originally belongs to older Yugi) & Kuribon]
Hey wait.. that makes Yugi have two duel spirits right? And Dark Magician plus Dark Magician Girl always respond to Atemu in duels & others..I guess they have many other duel spirits, so as the other two. At least they have a common fuzzy friend who keep appearing ^^.
Well that's all for now.I will be happy if you can contribute and list more similarities or PM to me. I will add them with your names aside
Villains..hmm I will do them too one day, I can't remember much if they have similarities but will help too if any of you have.
