Xiruk is the nobody of Riku, created when Riku reverted back to his original body in Kingdom Hearts II. When Riku accepted the light and darkness into his heart, fragments of both created Xiruk. After Riku took on the body of Ansem, his original body was inhabited by Xiruk, giving him the exact appearance of Riku, except taller with black hair. When Sora and Riku returned to Destiny Islands Xiruk laid dormant in the infinite darkness of the, now destroyed, The World That Never Was. Two mysterious voices called out to him, and he awoke. He then used his dark powers to find a way out. He then found himself in The Realm Of Darkness, alone.
Xiruk, not knowing who he was and questioning his existence, sat on the shore. When an empty bottle washed up on the shore he suddenly had visions of all the events of Kingdom Hearts II, as well as Kairi's letter to Sora. He then wandered along The Realm Of Darkness wondering what the visions meant. He then found he could control The Soul Eater and The Dark Keyblade. He then had another vision of the closing of the door to darkness and the battle with Ansem. After thinking, he decided he had to keep going to find his answers.
He used his light powers to open The Door To Light, walked through, and found himself in The White Room of the Old Mansion. Xiruk then saw a picture of Namine and Roxas in spaces on the picture. He then saw that there was an empty space where someone was supposed to be. Xiruk wondered who was supposed to be in the third space. When he touched the picture he had a vision of Roxas' meeting with Namine. A group of heartless then attacked him and he fought them off without a scratch on him. He then declared he did not understand his existence, but he had to keep going if he wanted to know the truth.
He left the mansion and realized he was in Twilght Town. He walked out of the gate and found a picture of Hayner, Pence and Olette. He assumed if he could find these people, then he might find some answers. He walked around the town and was amazed at the sunset sky; it was something he had never seen before. Before he knew it, a tear of joy had fallen from his eye. During his stay at Twilight Town, he took on various jobs to test his skills. He eventually encountered Setzer and battled him in a match of Struggle. He had never felt so much intensity and fun in his heart before. He finally found Hayner, Pence and Olette, but could tell that they did not know anything about him.
He stepped through a portal using he dark powers and ended up in The Realm Of Light. He floated around and had another vision. He saw Riku in the Realm of Light and saw Namine talking to him. He then assumed he had a connection to Riku, although he did not know what it was. He opened another portal and ended up in Castle Oblivion. He journeyed to each floor and along the way regained a memory of Sora and Riku's experience in the castle. Xiruk then exited the castle and saw the three roads. He had a vision of Riku's decision to walk The Road To Dawn. "The Road to Dawn..." he said. He walked down The Road and was surrounded by a flash of light. This time he was on the place he'd been looking for: Destiny Islands.
He searched the islands until he found Riku sitting on a palm tree. He approach and called out to Riku. Riku was shocked to find someone who looked just like him. Riku asked who he was, to which Xiruk responded he was also trying to figure that out. Xiruk then claimed that if he battled Riku he would regain his memories. Riku agreed and two began to fight. As they fought, Xiruk began to remember where he can from and finally found out what he was supposed to do. He then had a vision inside his mind. He saw Namine and Roxas and said "I'm supposed to merge with Riku aren't i?" Namine and Roxas said yes and said that's why they awoke him.
They then said that the three of them were supposed to be together, as Sora, Riku, and Kairi are. Xiruk smiled and returned back. He then walked toward Riku and merged with him. Riku then felt whole and found Xiruk was his other half. Sora and Kairi then came and ask what happened. Riku responded, "I just met with an old friend." He then smiled and looked to the sky. It then showed Roxas, Xiruk, and Namine sitting together on the beach as Sora, Riku, and Kairi. It then shows the picture in The White Room showing the picture of Roxas and Namine, but also showing Xiruk in the third space.