It's shown from the very beginning that Anna is an introvert whose attitude towards others makes it impossible to make any friends or get close to anyone. She also feels alone in her own family due to them being foster parents (She doesn't even call her foster mother "mom" until the end of the movie). Anna feels isolated and alone, all at the tender age of 12. Moving to the country was an opportunity to try and make friends in a place where she doesn't have the baggage of being "that girl" and she still fucks up by lashing out at one of the local girls. She meets Marine after this and is able to open up to her due to Marine's character and the belief that they shared similar pains. This is the first time in the movie that Anna is able to open up to someone about all her insecurities (belief that her foster parents don't love her because they get paid by the government to keep her, how she hurts people around her, her loneliness, etc.) I don't really see it as romantic.
Hell, the main theme song of the movie even narrates Anna's loneliness and insecurities, not about her yearning for Marine.
There was never any "undertones" that they're whining about. Anna felt that she was unwanted by everyone around her, but Marine helped her realize that wasn't the case.