I thought that Xehanort's Heartless didn't have a real body, and that's why he took Riku's
That is correct.
And when Ansem's apprentice, Xehanort had tooken darkness and the darkness within his heart took form as a heartless. that darkness was strong enough to take a form of its own, as a HUMAN. But made of pure darkness. Now, do u get it.
This is exactly what my theory was to explain why Ansem (Heartless) looked human... but let me rephrase what I just said... In the first game's Ansem Report (written by Heartless An), it states that despite being an existance of only the heart, Ansem had not looked any different...
Yet in the beginning of KH1, Ansem was only a dark mist in a brown cloak. You see him as this again in Hallow Bastion, however he's now 'misty-er' (I don't know how else to explain it XD). I believe that Ansem had the body of darkness that you are thinking of, but I believe that since Ansem was in the Realm of Light, the Light slowly began eating away at his physical body, destroying the shell of darkness. Ansem was only left as an existance of his own heart, supported by Darkness to keep it attached to this realm so it wouldn't go to Kingdom Hearts to rest. So then Ansem had to take over Riku's body, for without a physical body, all the Darkness that kept Ansem alive would vanish and his heart would go to Kingdom Hearts. Ansem desired to go to the Realm of Darkness so that he could live without a host's body and acquire even power.
That's my theory anyways. I think it make quite a lot of sense, no? Anyone, tell me what you think: Yes, it could be true, or no?