Guardian
Even though he's an animal and not a human, Appa is wise and sees things they miss; he's always there in any times of need.
He was the one who picked Aang just as Aang had picked him; he had chosen to go and run away with Aang. The young boy hadn't roped him into anything, he had willingly flown the two of them away. Maybe it was his animalistic instincts, but he felt (in one of his five stomachs) that running away was right.
And he was there when Aang fell in love.
He knew he was the first one to see it; to see Aang's eyes and voice light up at her face and smile. Appa knew what it meant even before the boy, the person in love, did.
The flying arrowed-beast watched, when he wasn't busy chaperoning the chatty monkey and idiotic oaf from getting any hooks stuck in his fingers, how they would talk and interact with each other.
How Aang's eyes always trailed after her; how she would secretly smile at the attention.
Appa tried to help his often flailing partner, urging him with yells of encouragement whenever he said or did something wrong. Somehow, that girl always seemed to understand and forgive him, even when he didn't say a word and solely gave a small, sorry smile.
He was happy at seeing Aang so happy, so weightless in love and attention as he finally got her. When he was finally able to grab her hand and heart.
Appa always has, and always will be, there to help.
AN: Hells yeah Appa is a Kataang shipper. Don't even question it.