[magnuskn]

Yes, I know.
However, as I said, there are scenes and even whole episodes (like I said, episode 8) wherein Sheryl is straight up stealing all of the spotlight for herself with Alto while being friendly with Ranka despite semi-knowing Ranka's feelings for Alto at this point.

Yeah, you're leaving out that Alto explicitly told Sheryl that he had no feelings for Ranka in episode 5. And while Sheryl still denied to herself that she had feelings for Alto between episodes 6 to 11, they were definitely there. I don't see why she would have to step back and let Ranka fumble her way around.

And ironically, the writers are self-aware of their choice because Nanase points it out in episode 8 (again, prior to episode 11): Sheryl is an outsider like Ranka who knows nothing about their world and bursts in and decides to act like a primadona.

Oh, please. Nanase was more into Ranka than Alto ever was. She is the worst judge of character in relation to everything regarding Ranka you could find.

The biggest issue I have in her character is like all characters in Frontier: Their personalities or motivations change by episode. For instance, you're pointing towards episode 11 as a defense, but do remember earlier in the episode wherein Sheryl discovers Ranka is planning on inviting Alto to her concert with front row tickets for his birthday. Even though Ranka planned for his birthday in advance (a little bit) and Sheryl herself had never once asked him or his friends when his birthday is, she once again decided to try and steal the spotlight last minute.

Once again, Sheryl already was very much into Alto at that point, though she only admitted it to herself at the end of the episode. She wanted to give him the perfect birthday gift and actually thought what he would like to do. Ranka instead came at it from the angle what she wanted to be a good birthday gift for Alto. Sheryl didn't owe Ranka anything and actually gave something important to Alto.

I find her characterization and its progression to be extremely consistent throughout the series, same wih Alto. Ranka is much more spotty, which is why she got particular attention in the movies, while both Alto and Sheryl suffered some pretty significant downgrades in that department.