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It's been known that sometimes a person will fall for someone similar to their parent. Notice that the blonde merboy Melody befriends has a Nice Guy attitude like her father and also sports a green tailfin like her mother (when she's a mermaid).
Notice Melody's reaction when the merteens invited her to hang out with them: She's speechless. Then, remember her (initial) with the human teens. This is perhaps the first time that Melody has felt apart of something and could be with people like her.
Why was Morgana so good at manipulating Melody? Aside from Morgana being an expert at manipulating and Melody being na´ve, there's also the one thing they (initially) have in common: Mommy Issues.
Notice that Melody has accidentally bumped into blondes of teens from both of her heritage: Human and merperson. When she bumped into the snooty, blonde girl you can see that they have met before. When Melody bumped into blonde merboy, he didn't know her. In a way, both of those times symbolize something:
The meeting with the snooty, blonde girl represents how much Melody is depressed about her life as a human teen, since she's so different.
The meeting with the cute, blonde merboy represents that Melody can live a new life and be happy and accepted.
The wall. It was built to make sure Melody never ventured out into the sea. Ariel had a valid reason for doing so, but Melody, obviously, didn't know. In a way, the wall also represents (to Melody) that Ariel is keeping her daughter from being herself.
Melody is a mer-human (half human-half mermaid). She is the physical representation the union between two different species. And at the end, Melody is the one to destroy the wall- the wall that separated these two groups for 12 years. The first mer-human reunited the union between two groups.
Yellow is the color symbol of happiness and joy. Notice there are three yellow things that Melody is connected to in the end of the sequel:
The yellow dress- The happiness she finally have at being herself and not having to feel "weird".
The yellow locket- The locket she received from her grandfather represents the happiness that she and her grandfather can have together as a family.
The blonde merboy- The (potential) romantic, happiness Melody can have with the cute, blonde merboy. His face just confirms that he likes her too!
Why did Melody faint after being caught by Dash, even though a second earlier, she was wide awake? Let's look at the list of what a 12-year-old girl, with a slim build has been through: Nearly died drowning, went running and climbing a castle by an evil sea witch, caught and desperately held onto a very heavy weapon, and again she faced a near death by being pushed by the same evil sea witch. It's been known that strenuous activity can cause a person to literally fall asleep or faint when they land on something soft.
How was Melody able to climb Morgana's tower so easily? Well, for about most of her life, she's been swimming in the sea and afterwards snuck back into her the castle. By climbing a window. Possibly everyday. There's also the fact that her mermaid genes give her some impressive physical qualities.
There's also the fact she inherited Super Strength from her mother's mermaid genes.
Mermaids are fish from the waist down, which implies they fertilize externally. For all of Ariel's life, she probably understood having children as a matter of squirting into a hole. Now imagine her reaction to going through humans' messy and painful way of doing it.