Ansem, Seeker of Darkness, sometimes referred to as Xehanort's Heartless, is the main antagonist of Kingdom Hearts and the "Reverse/Rebirth" scenario in Kingdom Hearts: Chain of Memories, as well as a supporting antagonist in Kingdom Hearts 3D: Dream Drop Distance. He is a member of the True Organization XIII and the Heartlessof Terra-Xehanort, and as such his appearance and personality are primarily derived from Master Xehanort, and he retains his original form's "shadow".
Ansem was created when Terra-Xehanort, masquerading as Ansem the Wise, reawakened his sleeping memories and split his heart from his body, creating a Heartless; this Heartless, also named Ansem, appears to have characteristics of both Emblem and Pureblood Heartless, and it is unclear exactly which category he falls under. However, because Terra-Xehanort did this of his own will, he retained his memories in his new existence, and gained the ability to control the other Heartless. With their power, Ansem attacked the Realm of Light, intending to have the Heartless consume the hearts of every world and lead him to Kingdom Hearts.
Ansem has not appeared yet in either the Pooh or Logan Adventure series, but he is a major villain in the Sora's Adventure series, manipulating Ralphscoe as part of Master Xehanort's schemes to destroy the Keyblade gang. He is also Mistress 9's blood brother according to the Pooh's Adventures family tree canon.