I don't know any of those characters, except by fandom osmosis, but it sounds like ATLA has similarities with FMA - lots of 'magic' users, but also lots of normals who hold their own.

And yeah, now that you mention it, I think Mei Changsu is basically... cosplaying... this sort of character, rather than actually exemplifying it. He doesn't have martial arts skills (anymore) or political power (at least of the open or inherited sort), but he does have insider knowledge and is definitely not the commoner he purports to be.

Now I kinda want a fic where still oblivious!Jingyan becomes suspicious in part because Mei Changsu is too unbelievable as a commoner. :D Jingyan's out on battlefields and inspection circuits and such; he probably has far more experience with commoners than anyone in the entire royal family, and he just gets this prickle down his spine sometimes when Mei Changsu is demurring too much or laying on the 'Your Highness'es too thickly... The commoners he knows are individuals, but Mei Changsu strikes him as a noble's idea of a commoner - too deferential, too perfect at remembering honourifics, too ready to bring up his own status in a deprecatory way. :D