I think they made Movie Sheryl flawed in her tool against the Vajra and spy dilemma, dealing with real consequences and some bittersweet karma outcome. This is how they should have gone with TV series Ranka. None of that magic-godly whitewashing bullshit. Sheryl, Alto, SMS, all of they should have stuck with Ranka, but I would like her more aware of her actions too.
Movie Sheryl is an actual spy from Macross Galaxy, a place she seems to love enough to pay for its rescue with her credits in the film. She’s aware of her disease (whether if they infect her with it or she was already infected, it’s not explicitly said. Since it seems to ruin their plans, perhaps its the latter) and she signed up in her role as Fairy until they find Ranka. We don’t really know how much she knew about the Galaxy take over (probably nothing of this), but she seemed seeking Ranka out of the memory of her grandmother’s research and beliefs.
And she also feels a strong love and loyalty for Grace, similar to how Ranka feels towards Ozma (Aya Endo empathized this love she feels like a child to a mother).
However, she also knew that the Vajra would attack Frontier sooner or later (Brera apologized because they attacked sooner than they thought). Was she trying to stop this or to use the Vajra to draw Ranka out? We don’t know (like Universal Bunny, Sheryl’s very controversial in the movie). Probably the former. She also feels this crippling loneliness. Maybe because he alone isn’t implanted and, while she might care for Grace and Brera, they are controlled. It must have been terrible for her watch them all this time.
Even though she’s dying and the only hope to save her is harvesting the life of her rival in song and love (remove an obstacle), she feels nothing but relief when she’s arrested. She accepts all the charges, the false and the truthful ones. She wants to pay for them. I really like this. Then her high point is to show that, even if she had a turmoil earlier and she has only spent three or four months in Frontier, she betrays the only people who could save her to keep Ranka safe. Then she is willing to die for Ranka’s sake.
I really like her character progression and I would have like to watch more her thought processes. She is more like the Mad Scientist’s Beautiful Daughter type. We know she means well in her feelings and thoughts (Ranka and Alto felt them), but she’s caught in bad circumstances.
In regards of the triangle, it was less, IMO, who Alto loved. It was very obvious to me Alto was drawn romantically to Sheryl through the movies. The scenes with them are charged and contrasting. But he couldn’t trust her, he trusted Ranka, his best friend, but he didn’t love her. In the end, he was able to trust Sheryl and unable to change his feelings for Ranka.