I actually think the character who thinks the most about justice is garrus— samara has a code she ruthlessly adheres to, but garrus spends three games trying to figure out what his code is or whether such a thing as a code can exist in a galaxy so messy and so determined to subvert it. His comment about wanting to see the world in black and white is the comment of someone wanting, desperately, to see the good rewarded and the evil punished, and someone genuinely troubled by the fact that every day the good are killed and the evil walk free.

but I wouldn't call him an avatar of justice; that's just my meta. garrus thinks the most about justice, but it isn't his defining trait in the way that samara is entirely defined by her justicar code.

I'd give garrus loyalty, for sure. loyalty, or maybe faith. He doesn't become shepard's "bro" until ME2 but even in the first game he thanks Shepard for bringing him on the crew; he tells her he'll do anything she asks him to simply because she asks; and then, in a particular bit of "show don't tell" the best part is that in the next game he joins her mission simply because she asks. Even when he expresses disagreement or discomfort with a decision Shepard makes (not immediately shooting dr saleon, working w Cerberus, or saving the geth over the quarians) his faith in and loyalty to Shepard, herself, remains rock solid. Here is a character so wracked by doubt, so conflicted over so many things, and yet the moment Shepard earns his loyalty and respect he never once considers revoking it.

The VS can whine about how hard it was losing Shepard, Liara can do a million questionable things in the name of her grief, but I guarantee you it hit Garrus the very, very hardest.

As for ME2 characters... I'd say jack is something to do with power. There's a deep, tragic irony to the fact that one of the most powerful biotics in the galaxy is constantly put in situations where her power is taken from her, that in fact she gained powerful skills at the cost of her own personal agency. I'd give thane redemption, though that's pretty well telegraphed in his dialogue. Miranda I'm still thinking about.

Also, is Shepard an avatar of victory? I'm almost inclined to say that Shepard is actually an avatar of sacrifice. Victory is achieved through sacrifice so often in mass effect, but I think the important part is that sacrifice comes first: Shepard not only decides what is and what is not worth sacrificing but at multiple points in the trilogy offers herself in sacrifice.

I had a lot of thoughts about this, apparently. This was good food for thought. Thanks OP!