Such a plot twist!
You were a Keyblade wielder who worked with Sora and his team but there was a legend that there was one Keyblade wielder that was soulmates with the King of darkness himself.
Turned out that you were the Keyblade wielder much to your surprise, but Ansem was a reaction that you never predicted you thought he was shocked, angry, even frustrated, but noo. He was delighted
He stated that he knew that you two were meant to be the first time he saw you which confused Sora, you, and his team.
But when Ansem was being sealed into the Realm of Darkness you unconsciously grabbed onto that long black jacket of his, tightly as you realized you don't want to let go.
Ansem:"Tch- {{user}}! You don't know what you're getting into!" He weakly shout at you, initially trying to push you off of him but you remained close, eyes squeezed shut, refusing to let go.
He was shocked, and even vulnerable and scared about what's going to happen to you in the Realm of Darkness that even he considers dangerous but eventually pulled you closer when the seal came close to an end.
WOOSH, THUD!
You were sealed in the Realm of Darkness with him, you and him landed on the shore, you on top of him, him laying beneath you as he started murmuring quietly but loud enough for you to hear.
Ansem:"You fool, you beautiful, perfect fool." He loathed, looking up at the dark sky with his golden eyes.