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The Avatar feels as though he has failed on the day that he has to congratulate the last waterbender of the southern tribe, upon finding out that she is carrying the fire lord's child. Not that he's failed as the avatar, because the union of the two nations has helped bring back peace, but as Aang. Aang failed somehow, and now he has to congratulate Katara when he hears she’s pregnant with Zuko's son.
He's not really sure when he failed. She just told him that she saw him as a friend, a little brother. Then later she told him that she couldn't keep traveling with him, she had her own things to take care of. Next it was her writing about a duty she did for her people. Finally he saw her smiling face again, and she was smiling at Zuko. He saw her waterbender's hands again, and they rested on her stomach, next to Zuko's fiery ones.
He wasn't sure why he failed. Maybe it had something to do with the fact he was twelve when they met. Maybe it was because she had spent so much time as his teacher. Maybe he just wasn't good enough for her. Though he still wouldn't believe that Zuko was either. That day, she was in a blue robe, trimmed fire nation red. She had the crown of the fire lady in her hair, and her fingers intertwined with his old enemy's.
When Aang received a message via fire hawk upon the birth of the Fire Lord's son, he sent back a congratulations and a rattle made of walrus-whale tusk. A traditional water tribe gift for the crown prince of the fire nation. He told himself it was about balance between the nations, but he knew that was a lie. The next two children received air and earth nation gifts, so that he could keep up the pretence.
He was at the palace for official avatar duties, but taking a break and visiting Katara's kids when the eldest son did his first bending. Aang bust out laughing. The crown prince of the fire nation was a waterbender.