In the first episode of the English anime, Yami defeats Seto Kaiba and uses his power to break his mind. Kaiba is able to recover.
He is slightly reserved, as he doesn't reveal his existence to Yugi or his friends. After Yugi admits to thinking "another Yugi" exists inside him, his friends become aware of his existence and accept him as a friend.
Yami's pride is his greatest strength and weakness. Initially he refuses to lose games, without considering the consequences. This has caused him to almost kill Seto Kaiba by attacking him while he stood on a ledge and would fall if his monster was attacked and Yugi would have won the duel (in the Japanese version, Kaiba threatened to commit suicide should he lose). Yugi holds him back from such acts at times, although not always successful. Yugi becomes afraid to let Yami out for a while, because of the harm he could cause. During his duel with Rafael, Yami, albeit with great hesitation, eventually ignored Yugi's objections and used the Seal of Orichalcos rather than lose. Ironically, it was this action and Yami's subsequent corrupt actions during the duel would lead to one of his few defeats and the capture of Yugi's soul by Dartz. Yami expressed great grief and guilt following the battle due to Yugi's willingly suffering, which were because of Yami's actions. Learning how to lose becomes an essential part of Yami's journey, which he is tested in the Ceremonial Battle.
Yami is also wise and intelligent. He knows all the rules and details of how the game is played. Yami is also capable of reading the ancient language of the Egyptians, figuring out the riddles or clues and what they mean.
Yugi possesses the Millennium Puzzle, which allows his body to bear host to the spirit of the Puzzle, Yami Yugi. Yugi was initially unaware of his existence and control of Yugi's body would involuntarily shift to Yami, when Yugi or his friends are wronged.
Yugi is part of a close-knit group of friends: Joey Wheeler, Tristan Taylor, and Téa Gardner as well as Ryo Bakura and (In some cases) Mokuba Kaiba. Miho Nosaka is also one of Yugi's friends in the first series anime. Yugi's friends often cheer him on from the sidelines, and Joey has dueled at his side. Yugi has been friends with Téa since childhood and only sees her as a friend. Téa has been shown to have feelings for both Yugi and Yami Yugi.