Complete Monster: Haine is a sadistic child and the leader of the Cult of Apophis. In his backstory, he is shown to have Mind Raped most of the other cultists into becoming his servants and in the series itself he takes a sadistic delight in watching people suffer in Shadow Games, whether they be his own men or not. His greatest onscreen cruelty comes when he tortures Marcus and outright murders another boy named Sylvestre (who had no business in the One Day War and simply happened to be in the wrong place at the wrong time). Then broadcasted the duel to the other students of East Academy in order to intimidate them. Unlike the other cultists who were brainwashed, or Apophis which is simply fulfilling it's purpose. Haine enjoys hurting people.
David also seems to be gaining a large fanbase, thanks to his Wild Card status.
Roxas is also quite popular among readers despite his comparatively minor role.
Fridge Brilliance: As shown before, Duelists' decks have some interesting links to their personalities.
The Ax-Crazy cultists often use decks that reflect an overwhelming offensive or control strategy, which relates to their desire to make the return of Apophis occur as soon as possible.
Art Caltenrich uses a Heroic Deck, reflecting his Heroic Resolve and Well Intentioned drive for greater power and esteem, to defend his loved ones.
Marcus Kyleman uses a tactical Alien deck, which monkeys with monster attack points and effects, reflecting his strategical mind and thinking patterns.
David Kinrab is apparently a sorcerer of some kind, and regardless what his alignment ultimately is, its clear his spellcasters focus on eliminating the opponents' options in tandem with his spell cards. He also uses some of Yugi's cards to reflect on him being a fanboy (at least apparently)
Apophis uses three decks, all of them reflecting on Its nature as an inhuman force of destruction.
Mort is a rather odd young man obsessed with diseases and plagues in an academic fashion, and his deck involves using Zombie World to turn everything into Zombies, and then reap advantages from that.
"Dark Angel" Lionel uses powerful Angels, either Fallen or otherwise, in his deck, and his status as a professional is reflected in how he is able to get them out one after the other with either traps or the Court of Justice.