Ansem lied about being Ansem when you discover that the real Ansem was actually DiZ, who chose to throw his name into a bin and take up the title Darkness in Zero, because he was mad and on a quest for revenge, so you can't go around being called Ansem as opposed to something edgy, but then Christopher Lee realized how incredibly stupid that name sounded, so he abbreviated it to DiZ. The Ansem who is not Ansem, was actually Xehanort, only that he is not, he's actually Spock's heartless, who had chosen to stab himself in the chest with a giant key so that he may transfer his heart into Terra (who is the Keyblade's chosen one, along with the other millions of chosen ones that are now dead and still live) to become Terranort, also known as Xehanort who isn't really Xehanort, so that he may unlock the door to Kingdom Hearts or start some sort of war where people whack each other over the head with keys, but then Terra-Nova decided that wasn't quite enough to fullfill his goal so he threw his heart away.. again, not entirely sure how one manages to do that.. but anyway, then he became a heartless and a nobody who is also called Ansem, rearranged with an X unto to it, which made him feel pretty good about himself, I imagine, as he's managed to compile a new name together from the old one that sounds at least plausible to pronounce; SEM-NAS.
So Xemnas builds an Organization of 13 losers.. uhm, nobodies, so that he may create Kingdom Hearts from a bunch of hearts collected by the protagonist, because that's what Kingdom Hearts really is.. It's a bunch of hearts collected together in one place, except when it is a Door to Darkness, but not really, it's actually light within that same darkness, only it isn't, Kingdom Hearts is actually the moon of some desolated wasteland. Xemnas dictates that collecting hearts together, that is to say, lie back, do nothing, and let the hero do it for you, will grant him what he really wants, a heart -- So maybe he wasn't such a bad guy after all, except when it turns out that it was all a cover up for his true goals of wanting to bring back Spock-Nort, who is also him, because he's part of Spock-Nort you see, as well as Xehanort (the one who isn't Spock-Nort). Spock-Nort somehow predicted an extremely unlikely series of events would transpire to bring him back.. Defeating an indvidiuals heartless and nobody somehow restores the original vessel, which leads to Spock-Nort's ultimate goal of creating 13 Spocks to rule them all. He'd talk about how he'd use time travel to gather 13 versions of himself into one place so that they may all become him, because if there's something Spock appreciates more than Darkness.. it's himself.. and thus the story continues.