"Ansem" appears in Kingdom Hearts as the game's main antagonist. Devoid of a body, he watches Maleficent do his dirty work in gathering the Princesses of Hearts. Once Maleficent served her purpose, Ansem possesses Riku's body to regain a physical presence while dealing with Sora at the End of the World. Although his goal of reaching Kingdom Hearts is realized, the light behind the door to Kingdom Hearts destroys him, causing most of the worlds he destroyed to be restored while the others entered in a state of sleep. But in Kingdom Hearts: Chain of Memories, Ansem is revealed to still exist and attempted to take over Riku again within Castle Oblivion. However, Riku subdued Ansem and eventually uses the villain's power before the essence of Xehanort's Heartless was purged from him prior to his battle with Xemnas.

Ansem reappears as one of the main antagonists for Kingdom Hearts 3D: Dream Drop Distance, having being brought from a moment in time by Young Xehanort. Ansem takes advantage of the Sleeping Worlds to mess with Riku by attempting to open the youth's heart to darkness again. After being defeated by Riku in the Sleeping World version of the World That Never Was, later appearing in the Organization's attire, Ansem attacks Riku and Mickey to hinder them from interfering in Xehanort's attempt to convert Sora. By that time, it is revealed that Xehanort's scheme included Ansem making use of the bodiless form to make contact with Young Xehanort to enact the back-up plan.

Ansem is voiced by Akio Ōtsuka in the Japanese version of the series. In the English version, the character was originally voiced by Billy Zane in Kingdom Hearts and now by Richard Epcar in all subsequent releases/remakes.