Some things you should know first...
(Oh yea, I don't own The Avatar: The Last Airbender)
Long ago, just a bit after the last Avatar passed on, there were two Waterbenders that shifted peoples gazes of all Waterbenders... Yes, Waterbenders have been known as healers and defensive benders, but these two took Waterbending in a slightly different direction... They totally mastered the art of Waterbending after about fifteen years, which if I went into detail, would be nothing but long, grueling, dangerous, and deathly training almost twenty four hours a day, each and every day. In addition to being as skilled of Waterbenders as they were, they decided that the style was purely too much defense to make a difference when you got higher up in the ranks and all. So, though they didn't, (and couldn't) exactly learn the other bending arts, they still learned martial arts that could (If you most likely tried hard enough) could be candidates to the other bending elements. They got Earthbenders to come in, which astounded the Waterbenders, as they watched the two train their styles with and against each other.
These type of things included:
Rock Crushing (The event of an Earthbender bringing up earth and trying to expand it as hard as they can, whilst another individual is trying to crush it. Quite dangerous.)
Stream Making, followed by ground coverage (Earthbenders hurl boulders, while the other individual spins the same way as the oncoming boulder, and tries to disintegrate the boulder with as powerful of a jet stream of water as they can, shooting it along the ground, making water flow, then the ground around will be turning into mud, and the individual must overcome the earth and utilize the water to move the dirt over the trenches. Whew...)
Rock Smashing (Big, almost fist-like gushes of water are fired at rocks to break 'em)
Rock Slicing (Though the premise is similar to Rock Smashing, this was a great deal harder than that could ever be. Form a super-quick blade of water to slice through rock.)
(I'll end the list here... it goes on and on.)
Yes, they did many exercises like this all day, and all night, and that kept the friends together, closer than brothers, and strengthened many lives along the way. Among already knowing Tai Chi, they learned (and popularized with non-bending warriors) the styles of Pancratium, Buddhist Palm, and Muay Thai. They could actually manipulate water a little differently using certain moves at the end of the flowing cycle of water to make it's impact like iron. Pretty much soft stream turns to crashing waterfall. These two Waterbenders split up to prove that their bending could become just as great as any. They never forgot that peace is king for a Waterbender, but how can I say this... they kinda took on an Airbender personality. Anyway, eventually, once the word of how awesome Waterbenders could be spread, they confronted the fire nation, who had taken over not too long ago from the completion of their training. They both headed forward against them, shoulder to shoulder, taking down military personnel like they were scraps of paper. They both got to Fire Lord Ozai. There, they both died.
They did not die in vain. They were remembered by many, of different nations, as friends. All nations 'cept most of the fire nation, that is. Anyway, people now gave water a bit more credit since then... but it wore off after a while. Thus...
Later, about 80-some years ahead, two children were born. Three years apart. One is ten and the other is thirteen. Very weird things happened to them, and they also found writings from the two legends of the Waterbenders. So train they did, day and night. Now, here's where I tell you something interesting. Let us start with the younger one... this is the first thing. He was born mute, due to a training accident from his brother. His brother has told him that ever since he was able to comprehend words, but when he kept telling him, he only felt worse because he could never know what his brother would sound like... He is exceedingly intelligent though, far beyond his years. He knows exactly what he wants, he knows things others never would, he knows how you feel... he just can't tell you, for only speech could accommodate what he would have to say...
His name was Irkalla.
His brother on the other hand, was fine, all up 'til he turned eight, ha ha! Trained and trained, trained with his brother, day in and day out... super-determined this one was, and he wasn't a dummy in his efforts either. He was imbued with the same mission as the two before them. To make a name for Waterbenders, and seeing as his brother was always with him in whatever he did, that became his goal too. Now, what happened was a shame. Now, Irkalla went home every night, but his brother, Mizou, slept outside next to the stream, against the wind in various positions almost every night. Eventually the wind and water going in his ears steadily made him go deaf. Yea. He still clings on to the memory of the sounds he already knew, and made, not ever forgetting them, or how to make them. After a while once they came of age, they decided to go and find the Avatar and do great works with him. Now, let's get this party started.