Can Riku sense what your character truly is? - Sure!
If so, what are they and can he tell just by a passing 'glance'? - He is a human and a Warrior of Light! I'm not sure how well that translates into KH concepts of light/dark. The Warrior of Light part is granted by crystals, so Riku might also be able to tell that Onion Knight's powers aren't from his own ability.
What level of "darkness" do they have within them? - Firmly on the side of light, though if he could job change in Vatheon, he can use dark energy as a Dark Knight (supposedly; for some reason they use... white magic??).
What do they smell like? - BOY. Umm maybe some fire and brimstone and grassy fields, since those were the places he was at last before he arrived in Vatheon.
What is your character's tolerance to dark magic? - Despite being a Warrior of Light, I don't think he has any particular tolerance or intolerance to it. So, neutral.
Can he break privacy laws and just teleport to where your character is? - SURE. He's pretty used to not having privacy.
Can he go Ansem on your ass transform into "Ansem" on them? - GO AHEAD just give me warning if that's gonna happen (though I imagine it'd be unlikely).