What sucks is that a fast character appeals to me aesthetically. Parrying/riposting/dodging/backstabs.
Aesthetically? Like, just looks? Or do you mean by moveset?
Well, either way, you can probably have your cake and eat it, too. You could use light armor, and switch between a heavy and a parrying shield, situation dependent. Same for weapons.
My thief is now being spec'd to tank with a shield and spear, but every once in a while I'll throw on the thief garb, a parrying shield, and knife. Works just fine. He'll probably get some magic, too, at some point. I'm deliberately ignoring any min/max'ing with the build, and just doing whatever floats my boat.
But even when he's all loaded down with armor and a heavy shield, I still get a lot of backstabs. I get them by moving or dodging behind an enemy when they're in mid-attack. Nothing complicated.