Alright, we know that Aang likes Katara, but just to prove it here are some signs
Disclaimer: I do not own Avatar: The Last Airbender
Note that this is not in order of the series.
'eats cookie and thinks'
(I have added three more clues! YAY!)
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Clue 1: The Fortuneteller: Aang bounds for joy when he overhears who Katara is going to marry
Clue 2: The Fortuneteller: After Aang had given her the made-up necklace, Katara said they were only good friends. Aang saddened.
Clue 3: The Cave of Two Lovers: Aang blushes like mad when Katara touches him for the waterbending lesson.
Clue 4: The Cave of Two Lovers: Aang panicked when Katara suggested they kissed. (seriously, how does Katara not figure it out?)
Clue 5: Bato of The Water Tribe: Aang sees Katara's necklace is in Zuko's hands, does everything he could to get it back
Clue 6: Bato of The Water Tribe: Katara kissed him on the cheek. He blushed like a girl.
Clue 7: The Boy in the Iceberg: Aang flies around on his glider, trying to impress Katara, and bumps into an 'ice wall'.
Clue 8: The Serpent's Pass: "I thought I was trying to be strong... but really I was just running away from my feelings. Seeing this family together, so full of happiness and love, it's reminded me of how I feel about Appa... and how I feel about you."
Clue 9: Various episodes: It seems that Katara is the only one who can stop Aang's Avatar state. Ever wonder why?
There are Probably countless more, but these are most of the main clues.