Final answer:
Aquata is complaining to their father about Ariel's behavior.
Step-by-step explanation:
The student complaining to their father about Ariel's behavior in the dialogue is Aquata. In the question, Aquata is expressing frustration that their father treats Ariel differently and lets her get away with everything. This can be seen through the use of the phrase 'you let Ariel get away with everything'.