I don't own Avatar.
Aang sat by the fire, looking through its red depths to the other side. Across from him, Katara was mending a shirt of his which he had ripped in their fight against the swamp. Behind her, Sokka stood silently, looking up at the sky and carefully fingering his boomerang.
"Why do you do that?" Aang asked suddenly. Sokka turned abruptly, startled by the noise. When he realized who spoke, however, he asked. "Do what?"
Aang breathed out impatiently. "Play with your boomerang. You do that a lot. Especially when you look at the sky."
Katara looked up and gave Aang a stay-out-of-it look. Sokka sighed and continued sub-consciously fingering the weapon.
"Dad gave it to me." He said quietly. "He wanted me to be a warrior, like him. I wanted to be one too, but-" Sokka stopped suddenly and turned around again.
Katara sighed. "Sokka used to Waterbend. Did you know that?" Aang shook his head, feeling sorry he had ever brought up the subject. He could feel a sad story coming on.
"He was one of the best Waterbenders in the village. He taught me most of what I know." Katara flashed Sokka a loving smile. "He didn't want to give it up."
Aang was confused. "Then what happened? I don't get it. If Sokka liked it, then what's the problem?"
"The problem is you can't be both a Waterbender and a warrior. They both require discipline, and time." Sokka said this quietly, surprising Aang by not going into one of his almost constant rages. "We needed a warrior in the village-badly. Dad and the other men had to leave for the war. Dad gave me this just before he left."
Katara smiled. "What were his words again?"
Sokka smiled too, but his was sad. "he said "Sokka, you have to be a man now. Don't let anything happen to your sister or your mother. I'm counting on you to accept this responsibility." Then he left. Just like that."
The group was quiet for a few moments, no one daring to say anything lest it offended somebody else. Surprisingly, it was Sokka who broke the silence. "I didn't do a good job of that, did I? Mom died three months later and you-" here Sokka nodded at his little sister. "You have been in more dangerous situations then I care to count."
Katara bowed her head, tears welling up in her eyes as they did at any mention of her mother.
Aang was still looking at Sokka perplexedly. "I don't get it. Why did you give up Waterbending? And why didn't you tell me? You could have helped me learn!"
Sokka sat down next to Aang and put his arm around him, the closest he had ever come to behaving brotherly towards the young Avatar. "I don't need anymore responsibilities, Aang. To help you would give me more."
Suddenly, Sokka stood up. "I must sound so selfish, talking about myself all night." His fingers were back at their old place on his boomerang, tapping it more energetically then usual.
Katara went back to her sewing, but not before saying softly. "You're not selfish, Sokka. A little stuck-up, a little clumsy, but never selfish. Not after what you gave up for me."
One more chapter will do it, I think. It won't be very long. This is just my explaination for who Sokka really is.