calm down or your blood pressure will skiy rocket.
Meyrin, though yes for the 1st 36 episodes did nothing but sit on the birdge, but after that her character development took off.
She literally risked her own life (and dignaty) to save Athrun. She helped protect Lacus from a bunch of assassins. She helps take care of Athrun after he reinjurs himself and the 2 grow closer. And there is a reason why she was assigned to be the CIC officer of the ZAFT flagship, because she's damn good at her job: one of the best. with this experiance she then helped coordiate and guide the Eternal on the attack on the Massia base after the Archangel stopped covering them to go attack the Requiem. She then won Athrun in the end.
All in All Meyrin got more character development in 12 episodes then any of the other female characters. all Lunamaria did was, launch out of the Ship and blow stuff up, and kiss the man who "killed" Athrun and Meyrin. she acted more like Flay.