"Describe the following Star Wars character WITHOUT saying what they look like, what kind of costume they wore, or what their profession or role in the movie was. Describe this character to your friends like they ain’t ever seen Star Wars."
This is a test that RedLetterMedia and should a character pass it then they pass the character test, here's an examples from The Legend of Korra.
Asami is kind, intelligent and caring. Despite all the crap through and getting repeatably stabbed in the back by those she trusts, Asami always looks for the best in people. However, she makes it clear to those who have betrayed or hurt her that they have to earn her trust before they can regain her friendship.
She is VERY level headed, almost never rushes into a situation without some kind of plan. But when push comes to shove and she is forced to go in without a plan she is more then capable and ready to kick ass. She is loyal to a fault and never once thinks about hurting those she cares about and when she does hurt someone she cares about, be it directly or indirectly, she Women's up and admits her mistake.
Now try talking about Mako without bringing up his relationship with Korra or Asami. See the problem? His whole character revolves around his relationship with his love interest to the point that even his relationship with Bolin is more of a cliff note then anything else.
The fact that Mako's character revolves around his relationship with Korra and Asami only becomes more glaring after Spirits as he continues to be part of the main cast and yet doesn't really do anything and most of what he does could have been done by anyone and nothing would have to change. Here is a list of everything Mako has done over the course of the series.
He gave Amon a electric shock, suspected Aiwei of being the traitor, relayed a message from Zaheer to Korra, killed Ming-Hua, refused to watch Wu which resulted in the idiot getting captured and destroyed the Colossus' power core. Every single one of these events could have been done by anyone, even destroying the power core could have been solved by had Kuvira not "Overwritten" it. Hell at this point the Colossus wasn't a real threat anymore as Korra was kicking Kuvira's butt and Lin and Sue had already dismantled the main weapon.
And it gets worse, as said on Mako's trivia page Mako originally did have a arc like the rest of the main cast but unlike Korra, Asami and Bolin it wasn't him overcoming any personal demons but a romantic arc for with Fire Lord Izumi's daughter. Yes really, after two seasons of just being a love interest and one season of doing pretty much nothing Mako's final character was going to be him getting involved with a character we have never seen with only one season to set up said relationship.
In other words, the same exact romantic arc he was given in Air that was one of the most heavily criticized points of the that season was going to be the very thing they were going to give to him for the series finally.