Not You / Tú No - "Who...are you...again?...it's weird...it feels like I'm forgetting something really important...

Basically, everyone's memories of Xion vanished when she died (except of Roxas, Axel, Riku, Xemnas, and Saïx) which I personally find a really idiotic excuse for the plot

in Kingdom Hearts coded its revealed that Xion is one of the people that, while gone, is still connected to Sora's heart.

in KH: Dream Drop Distance she appears during the segment where Sora finds himself dreaming

as well as during the endgame with Riku

It's… heartbreaking