I've explained on my patreon how the story works but maybe I didn't mention it here on itch.
After Mars comes back with Elena and player regains control.
You'll be able to play through Sera, Ceci and Elena content however you want.
But if you reach the end scene with one of them, it will advance the story to the Mars x Erin duel.
This part is not done yet, Sera and Ceci content is available in the latest update.
Next update is Elena's and after that the duel.
If you want to play through Ceci's content, check my latest devblog post.