Princess Yue
Princess Yue is the princess of the Northern Water Tribe who reside at the North Pole. She isn't a major character, but helped heaps during the short time she was on-screen.
Princess Yue was betrothed to Han (spelling?) before she met Aang and his friends. She obviously didn't like him very much. When Sokka first saw her, he was captivated.
He plucked up the courage to ask her "if she wanted to do an activity together" during a banquet to honour the avatar's return. Yue was very amused by Sokka's umm....lameness. The next day, she told him to meet her at some bridge that night. That night, she acted all weird and rejected Sokka and Sokka's errr... carved bear, no, it was a carved fish. Sokka was shocked and complained to Katara and Aang.
Days later, during Katara's speech about Northern Water Tribe's customs and how her gran-gran ran away because of them, Yue got upset and ran off. Sokka followed her. He found her at a bridge crying.
He told her he understood, and it was because of their status. Yue denies him and kisses him. She then explains that she was engaged and ran off leaving Sokka alone on the bridge.
Later, Sokka then attempts to cheer her up by taking her on a ride on Appa. After the ride, Sokka realises black snow drifting down. 
As the fire nation attacks start, Yue could only watch from the sidelines. After Aang comes back from doing everything he could, he said that he couldn't do it anymore. Yue told him he had to, because he was the Avatar. That night, she explained the legend of the spirits and Aang had an idea. He said he might be able to help by contacting the spirits, but he did't know how to get to the spirit world. 
So Yue takes him to the spirit oasis, where there are two fish in the middle, one black and the other, white. She watches over him as he enters the spirit world. Then Zuko attacks. Yue runs away to find help while Katara fights him.
Later, when she came back with Sokka and Appa, Katara regained consciousness and realised Aang was gone. They went to find him. Eventually, they find him and Zuko. They defeat Zuko, but Aang said he shouldn't die out here in the snow and took him with them. During that time, Commander Zhao takes action and heads toward the moon oasis, knowing about the moon and ocean apirit's mortal form. He intended to kill the moon spirit.
As they were flying back to the battle scene, Yue gets a headache and the moon turns red. The moon spirit was in trouble. She then explained that she owed the moon spirit her life, because when she was a baby, she didn't cry like normal babies. She was very sick and weak, and the healers couldn't heal her. Her father prayed to the moon spirit to save her life, and that night placed her in the Spirit Oasis. Her dark hair turned white, and she began to cry. That's why her mother named her Yue, for the moon.
When they got to the Spirit Oasis, Zhao was holding the moon spirit, prepared to kill it. Iroh intervened, saying he shouldn't meddle with the spirits and threatened him. Zhao softened, and let the moon spirit go. Suddenly, on an act of impulse, he killed it. The moon disappeared and Zhao fled, after Iroh started attacking. Aang, then combined with the ocean spirit and went into battle as a huge water god.
Back at the Oasis, Iroh placed the dead koi fish back into the pool and looked at Yue. He realised a connection between her and the spirit and said that some of its life was in her. She then realised that she could give back that life that the spirit once gave her. Sokka tried to stop her, but she shook him off, saying that it was her duty. She placed her hands on the koi fish and transfered her life force to it. 
She then fell back, dead, into Sokka's arms. The koi fish started moving. Yue's body disappeared and a her, as a moon goddess appeared in front of them. She said her final words to Sokka, kissed him and disappeared. The moon reappeared. The ocean spirit stopped it's rampage and separated from Aang, it then grabbed Commander Zhao and towed him to a watery death.
After the battle, Sokka is standing with Yue's father, looking at the sky. He said that he was proud of his daughter...and sad as well.