Riku never lost his Heart. More so than him being too strong for it, Ansem couldn't afford Riku's Heart and Body to separate.
What happened is that through the Darkness in Riku's Heart, Ansem managed to sneak inside, resulting in a situation much resembling Kairi and Sora - two Hearts in the same Body at the same time.
Ansem's Heart, being the stronger at the time, took over Riku's body and as a result, Riku's body changed according to how Ansem looked.
When Sora beat Ansem, Riku managed to regain control over his own Body - but his Heart never left his Body, thus resulting in neither a Heartless nor a Nobody.
Lol proof Kairi messed up Sora's view of things in KH1.
And Kairi can say the ever the heck she wants, everyone knows she doesn't care about Riku unless Sora's there to see.