Can Riku sense what your character truly is? Yes! He probably already knows.
If so, what are they and can he tell just by a passing 'glance'? Sora's "Nobody," but likely with a heart of his own. He doesn't even need to look.
On a scale of one to ten (one being Princess of Heart aka nada, thirteen fucking Xehanort), what level of "darkness" do they have within them? Roxas is a Pretty Princess who likes his tea with sugar and dressing up in skirts and His element is Light, but he can still use Oblivion. He's probably around 2-3?
What do they smell like? Twilight Town and Destiny Islands. Sea salt, cold keyblade metal, Light. His Twilight Town clothes might have scents of the plastic-foam of struggle bats and warm pretzel on them for a while. He'll be in those, usually. His coat would have a leathery smell.
What is your character's tolerance to dark magic? High. It depends on his ability to deflect and dodge at the time, but say he was infected with it, he would probably heal faster than usual, given his Light.
Can he break privacy laws and just teleport to where your character is? Yep!