*possible spoiler* ok so I dont understand the whole thing about the X on soras chest that ansem was talking about. And what about the thing about riku being soras heartless ? I dont get it.

Riku was Sora's dream eater since he was in Sora's dreams.

Then the X on Sora is how the Organization tracked him and lead him to The world that never was

Riku was sora's dream eater to rid sora's dreams of nightmares

 

The X was the organization's way of tracking sora

Thanks !

In more detail:

 

From the Kh wiki:

 

The Recusant's Sigil (異端の印 Itan no shirushi^?, lit. "Mark of heresy") is a symbol designed by Xehanort to represent the rejection of a Nobody from his original persona. It is incorporated into the new names of each of the original members of Organization XIII, and is derived from Xehanort's sleeping memories of the χ-blade.

In order for Xehanort to track his movements in the Sleeping Worlds, Sora was marked with the Recusant's Sigil, which took the form of a silver "X" on his shirt.

 

As for Riku:

Ansem: At the start of your test, when you struck out from that tiny island, you saw me there dressed in a robe. Immediately you knew something was wrong, so without even knowing it, you dove into Sora's dream.And you became exactly what that sigil on your back represents--a Dream Eater to protect Sora from nightmares.

And once again, from the Kh wiki:

 

Ansem reveals that Riku is actually a Dream Eater, and, over the course of the exam, has been purifying Sora's dreams of Nightmares.

The sigil on Sora's chest was meant for Roxas when he left the organization and when he created his own identity and denied his original identity. If Axel brought him back, Xemnas would have put it on him to show him as a recussant. Since he returned to Sora, Xehanort put it on Sora in place of Roxas and used it to track him.

 

Riku was Sora's dream eater and after seeing Ansem in his robe, he dove into Sora's dreams.

Sora had the X back in KH II. Same with Terra, Ventus, Aqua, Eraqus, and Master Xehanort.

I'm still wondering how exactly that X got on Sora's shirt in the first place.