There’s been a really good discussion of whether or not Mako and Bolin are pointless characters to Legend of Korra and it got me to thinking about how pointless characters exist.
The problem with these characters is that there story doesn’t play into the overall story arc. Take the Gaang in Avatar. Katara wants to be a better water bender partly because of her mom dying and partly because the Fire Nation ruined any chances to formally learn. Sokka wants to fight the fire nation and runs practices with the young boys in the tribe. Note that while they do go along with Aang on his journey, they had reasons for being involved before he came along and have aspirations that don’t include him.
Characters that aren’t going to be just one shot or even side characters need to have a stake in the story. Bolin and Mako don’t have any connection nor is one made besides the fact that Korra is their friend, she’s fighting Amon therefore they fight Amon. Asami has a bigger stake than these two since her dad is actively involved and she is conflicted between doing right and trying to make things right with her dad.
And even if characters don’t have a personal stake they should at least have stated other reasons for fighting alongside the main character. Toph, for example, while not having any personal stake did have her own reasons for joining Aang and that was to be away from under her parents’ control.
My own take is that when you have a cast of characters who are main characters, I should be able to follow their own story and not have it be completely intertwined with the main character. The Bending brothers did have their own goals during the probending arc but once it was over they really didn’t add much and it just highlights another weakness within the show.