Can Riku sense what your character truly is? yup.
If so, what are they and can he tell just by a passing 'glance' Silver, Riza and Touko are all human.

What level of "darkness" do they have within them?Riza is Riza She's got her ghosts yea, but she's more towards the light than the Darkness. Silver is as I have said, more than a few handfuls right now towards the darkness, but he's also on that 'cusp' of what path he'll really take. Spoilers: He takes the lighter path eventually.

Touko has a lot more Light than Darkness - she's very pure hearted, but she's still got darkness in her.

What do they smell like?Riza: Gunpowder ect. Military scents.

Silver: His pokemon...? IDK. Silver might smell of dusky scents too.
Touko: She smells of dragons. She literally smells of that and something rare, mythical (Zekrom and Victini). Under that I'd say there's definitely the rural town girl there.

What is your character's tolerance to dark magic? average human tolerance I guess...?

Can he break privacy laws and just teleport to where your character is? As needed, yea. make sure Riza doesn't shoot you, too!

Can he go Ansem on your ass transform into "Ansem" on them? As needed, yes.