>>4238206>The romantic rival that seems "right" in every way actually not getting with the protagonist is also bit of a romance trope in itselfI'm aware of it, but the anime didn't simply retread this particular trope, they set up the story as a typical shounen anime about a brave boy saving the world; up until the final third, the hero is the one who did all the fighting, solved all the problems, rescued the princess multiple times, even dated her.
It's a mecha anime and he was the only good guy driving a mecha.
And among others, we all know "the hero gets the girl", that's one of the most historic het tropes of fiction, and they obviously played this expectation up to eleven.
Himeko being such a passive MC only reinforced this idea.
Chikane on the other hand was just this petty, jealous presence that got in the way of the hero because of her connection to the princess, her role was the same as a possessive father in a boy x girl romance, only in this case it was a possessive friend in love with the girl, the third wheel who's expected to give up by the end.
The moment Chikane took things in her own hands however, all expectation that were so thoroughly built up were subverted in rapid succession, only then it became all about Himeko and Chikane, with the hero being the unimportant third wheel.
If you look at any yuri manga with a love rival, the story is usually still about the main couple from the start, at most the love rival is almost as prominent as the love interest, but he/she rarely outshines her for most of the work.