The Awakening

Berserk Mode is an unofficial term used to refer to the state of mind that Yūya, Yūto, and Yūgo occasionally enter.

Effects
This ability causes Yūya and his dimensional counterparts to go into a state of rage, and their eyes to glow. This usually occurs in a Duel invoking the Four Dimension Dragons where the dragons overtake their owner's mind, or when the user is sufficiently angered, and its activation is accompanied by an excruciating amount of pain. While they are in Berserk Mode, they are willing to harm or destroy their opponent in order to win the Duel. When they regain their senses, they generally have no knowledge of what happened while they were in Berserk Mode. The only exception was Yūgo, who is able to retain a glimpse of his memory in such a state. If they enter Berserk Mode while being merged by one of their counterparts, they become vulnerable to being influenced counterpart's own memories and desires.

Notably, Yūya's Berserk Mode is slightly different from Yūto and Yūgo's. Originally, instead of going into a blind, homicidal rage, Yūya is collected and cold, speaking no more than necessary. However during his Duel against the Obelisk Force, Yūya instead acted on Yūto's deep hatred for the Fusion Dimension for invading the Xyz Dimension and proceeded to defeat them ruthlessly. Additionally, his hair becomes wild and his pupils glow red instead of his entire eyes; in extreme cases, his hair sticks upward, his pupils glow white, and he is surrounded by a dark aura. Because he absorbed Yūto's soul and merged with it, Yūto can indirectly take control of Yūya, although both souls become increasingly vulnerable to a darkness born of Yūto's hatred. Once aware of the circumstances, however, Yūya has become capable of breaking out of this mode by sheer force of will.

History
Beserk Mode initially appeared during Yūya's duel with Strong Ishijima where, while his eye's didn't glow, he became serious and calculating and proceeded to defeat him. Berserk Mode appeared again during the Duel between Yūto and Yūgo. When Yūto Xyz Summoned his "Dark Rebellion Xyz Dragon" and Yūgo Synchro Summoned his "Clear Wing Synchro Dragon" their eyes glowed (Yūto's eyes glowed blue while Yūgo's were green). After being reminded by Yūya that he didn't want to hurt anyone, Yūto came out of Berserk Mode, but then lost to Yūgo a moment later.

Later in his Duel against Isao Kachidoki, Yūto appeared in Yūya's mind and the two merged, causing Yūya to enter Berserk Mode and duel ruthlessly. He summoned his Pendulum monsters and used them to summon Yūto's "Dark Rebellion Xyz Dragon" and won the Duel brutally. When Yūya regained his senses, he was met with concerned stares from the audience and disdain from Isao causing him to wonder what he had done.

During his Duel against Makoto Takeda and Ken Umesugi, "Dark Rebellion" appeared in Yūya's mind and he would have entered Berserk Mode had his ally, Dennis Macfield, not managed to bring him back to his senses before it fully consumed him. Yūya proceeded to summon "Dark Rebellion" and was able to remain in control for the remainder of the Duel and use it's affects to win.

After seeing the Knight of Duels students being turned into cards by the Obelisk Force, Yūto's memories of when Heartland was invaded and vengeful hatred towards the Fusion Dimension flooded in Yūya's mind, triggering Yūya to enter Berserk Mode again to take revenge on them. This time, Yūya's Berserk Mode took on a whole different level; Yūya became more brutal and even enjoyed inflicting damage to his opponents. During the Duel against the Obelisk Force, Yūya progressively became more malicious and violent, becoming surrounded by a dark aura, and ultimately he summoned "Odd-Eyes Rebellion Dragon" to defeat them all at once. After winning the Duel, he continued to his next target, Sora Shiun'in but was restrained by Gongenzaka, Mieru Hōchun, Michio Mokota, and Teppei Tairyōbata. Yūya only kept on yelling violently in pain until he snapped out of Berserk Mode. Mieru was able to determine that there were two hearts within Yūya, along with a terrifying darkness. Yūto's soul was the one who initially triggered the Berserk Mode, but the darkness eventually took over both souls and worsened the Berserk Mode.

When Yūya saw his friends being turned into cards by the Obelisk Force, he entered Berserk Mode once more and chased after Sora. After reaching Sora, Yūya recalled Yūto's words and managed to calm down.

Goaded by Reiji Akaba in their duel, Yūya briefly evokes this state before snapping out of it on his own.