It's not really integral in the story. We saw none of it after season one, minus one cut away scene from what the brothers were up to after the season one had concluded. It's was used as plot device for Korra to help learn air bending but give credit as the creators of the series did think of it before Korra was ever even a thing. I'm using it to make my character grow and put in two characters who will be rather fun for all the players as a whole to interact with as well. Hana is a prankster with a golden heart and is extremely friendly. Lee is the jaded guy with a honest and firm personality. They will contrast with other characters and not only improve Ren but others.

Regardless of how integral it is in the story here's the main problem with having another group of pro-benders. Mine are all going to be relatively active in the group. That means we're going to have six benders just by the two of us. Add in two more benders and a chi blocker. That's a rather large group of benders unless the others want to go and be a part of your group. If you think you can handle three other characters then by all means you can try. Not saying I like it or that it's going to be good for the roleplay, there's going to be a large need for bad guys for these gangs we're going to be fighting to actually be a challenge if everyone comes along, but I'm not the guy who's going to be the 'OP GM who doesn't let others have the same thing he has.'

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As for Xia I'm unsure about what you mean by explosive exactly. I'm relatively fine with most of it even if some of it seems like a bit of a stretch per say but explosive isn't a term I'd use. There's versatile benders who are jacks of all trades and or have multiple bending abilities such as earth bending and metal bending, power benders who are good at powering through things, and skillful benders who specialize in one are or another.