Souji Kusakabe was originally an human infant used in Negishi's experiments in hopes of turning all of humanity into Natives, turned into a Worm that mimicked version of Tendou in order to be used by Mishima in the testing of the Hyper Zecter. Because of his origins, Souji seems to be a kind and innocent person, but can become as equally cruel to those whom he can't forgive. Because he mimicked Tendou, Souji possesses the Dark Kabuto Zecter, a Zecter that mirrors the Kabuto Zecter. When Kabuto is carried to this dimension, the mimic Souji stands by Hiyori defensively, angered by the presence of Kabuto. Calling upon the Dark Kabuto Zecter, he changes into Dark Kabuto and prepares to fight Kabuto. During his first battle, he and Tendou initiate similar Rider Kicks, resulting in Tendou being defeated and flung back to real world. Dark Kabuto arrives in the real world through a bolt of lightning, and walks away with a childish look on his face.
He appears before Kagami and others, stating that he's going to eliminate the other self. The mimic Souji eventually meets the real Tendou and the two engage in battle. The real Souji decides to allow himself to be defeated and thus when Dark Kabuto returns to the universe Hiyori is trapped in, Renge can track the portal and find Hiyori. When Tendou is on the brink of defeat, Gatack saves him and manages to convince him to fight Dark Kabuto, saying that he cannot allow himself to be defeated like that. Kabuto, now renewed with will to fight, defeats Dark Kabuto using the Hyper Zecter. However, Hiyori appears after a solar eclipse and says that she is happy to stay with Dark Kabuto in another world. After doing so, Hiyori changes into Sisrya Worm and prepares to protect Dark Kabuto.
After Hiyori and Dark Kabuto return to their dimension, they meet up with Tendou once again, who says that Hiyori should come back. The mimic Souji asks Tendou if he can truly keep his promise of always being by Hiyori's side, and Tendou says he can't. However, he will protect Hiyori for as long as he can, fighting all those who do evil to the world, not because they are Worms. Soon after, Hiyori leaves and abandons Dark Kabuto, who later returns to the real world and assault Hiyori for her betrayal. He seen encounters the two Reiji Nogi clones as they offer an alliance, telling Dark Kabuto that he does not know who he truly is. Refusing, Dark Kabuto leaves in a frenzy after attempting to fight the Nogis. After returning to challenge Tendou, he is defeated by Hyper Kabuto. The unconscious mimic Souji is then captured by Mishima once again.
After being tormented by Mishima and Negishi, who both state that he is a failure, he goes into an outrage, attacking Mishima as he learns his true origins and why. Tendou appears and uses Clock Up to rescue his mimic. However, the mimic escapes once again, only to meet Mishima once more. As Mishima transforms into Gryllus Worm, Dark Kabuto charges at him and is defeated. After Dark Kabuto is captured once again by Mishima, it is revealed that they plan to drain his power and combine it with a fragment of the meteorite in order to convert the entirety of humanity into Natives. During the final moments, Dark Kabuto rushes out and captures Negishi, dragging him back into the explosion as Dark Kabuto assumes his true form, telling Tendou that he is leaving and ensures him with protecting this world now.