Although more commonly known as a good but mysterious person in Yu-Gi-Oh! Duel Monsters and the later portions of the manga, Dark Yugi ("Yami Yugi" in the 4Kids dub) starts off as a very dark, ruthless and manipulative gamer who relied both on incredible luck and dark magic to win "Shadow Games", in which the loser will be subjected to a terrible punishment.
Dark Yugi, in the Yu-Gi-Oh! manga, is an alias used by Atem, who began possessing Yugi when he completed the Millennium Puzzle, an ancient Egyptian artifact that promises a wish granted to those who solve it. He is almost identical to Yugi, apart from being noticeably taller and possessing a wilder hairstyle. Initially, he only appears at night at the beginning of the manga. Dark Yugi takes revenge on the individuals who attempt to abuse and exploit Yugi and his friends, challenging them to various "Shadow Games". His "Shadow Games" usually revolve around revealing his enemies' corrupt desires in the form of a "Penalty Game", an intensive form of dark magic that creates illusions to reveal the victim's true nature. For example, after defeating a greedy, powerful bully named Tetsu Ushio, the Penalty Game "Illusion of Avarice" caused Ushio to hallucinate leaves and trash as money. Sometimes, the Penalty Game does not involve magic at all, such as "Landmine Prey", in which a group of thugs are fatally electrocuted. This is Dark Yugi's poetic sense of "justice", which greatly differs from Yugi, with his "forgive and forget" attitude. Not until later in the manga does Yugi even become aware of Dark Yugi's existence, having no memory of anything that happens when Dark Yugi in in control of his body. Yugi's friends initially believe that Dark Yugi is simply another side of Yugi's personality before learning that the two are in fact separate entities.
As the manga goes on, Dark Yugi slowly begins to change into a kinder, more merciful person (following a duel in which he nearly killed Kaiba before Yugi intervened), though still being able to do more in the sake of "justice" for Yugi. By the end of the Duelist Kingdom arc, he stops using Penalty Games, believing that his continued use of them would prove that the Millennium Items had an evil will.
The closest depiction of the original Dark Yugi in anime is only seen in the 1998 anime, which is an adaptation of the original manga that was never dubbed into English. The majority of Dark Yugi's Penalty Games are illusions in this adaptation.
This version of Dark Yugi has his eyes change color from violet to red when he transforms, an aspect exclusive to this incarnation.
In Yu-Gi-Oh! Duel Monsters, Dark Yugi is shown to be far more merciful than his manga and 1998 anime counterparts. At one point, however, he near kills Kaiba in order to save Yugi's grandfather. The only penalty game shown here is "Mind Crush", which he uses on Seto Kaiba to destroy the evil within his heart, on the Imitator of Death (an evil clown hired by Pegasus J. Crawford and pretending to be Kaiba's ghost), as well as the Player Killer of Darkness and a thug who tried to assault Tea Gardner.
Apart from this, this incarnation of Dark Yugi is shown as being a much less poetically justified person. One exception to his kinder personality was during the "Waking the Dragons" Arc (in Season 4), when Dark Yugi's sadistic side surfaced. His dark side was first revealed when he played the Seal of Orichalcos in order to win his Duel against Rafael; under the Seal's influence, Dark Yugi ruthlessly sacrificed his monsters is order to directly damage Rafael's Life Points via the effect of Catapult Turtle, only to lose to Rafael's Guardian Eatos and nearly lose his soul before Yugi took his place. His dark side resurfaced while he was dueling Weevil Underwood when the latter decided to tear up what he said was Yugi's soul card (which was actually a common card) as a joke. Unfortunately for Weevil, Dark Yugi became enraged and proceeded to defeat him with his Breaker the Magical Warrior through the effect of Berserker Soul, repeatedly drawing monster cards to sacrifice in order to directly attack Weevil, continuing to do so well after his opponent had run out of Life Points until Tea snapped him out of it.