You will obviously have Tiny Cyrene and Adult Cyrene as playable charaters.
It will be like Phainon again. You start with Tiny Cyrene, and once you activate
her ultimate and deploy her territory, you'll have big titty Adult Cyrene. Like how
Phainon has his base model at the start of the fight, but becomes Emanator once
he ults. There's no reason for Hoyo to make you bond with Cyrene for almost an
entire year with her cute Tiny model only for them to make her adult permanently.
Her technique might even show her 2nd form in the overworld for a brief moment,
like how Phainon and Firefly show SAM. And even if it doesn't work like this, I'm
confident she'll be like Firefly. With her cute usual self in the overworld, and her
Adult Emanator form in battle.

Now that I type this, I remember people having the same worries with Firefly,
thinking she'd be inside the robot all the time, or the other way around: that
we'd never see the robot again.