>>535989219
Collection wise Haruna and Fuuka are limiteds on their second (third?) rerun.
PVP and DPS wise Haruna has fell off.
Fuuka is one of the three cost halvers of the roster.also makes it easier to use high cost units. (If you are into account building, that is). You can use her almost everywhere.
The others are more situational and Christmas Serina and Hanae are meme units with a unique mechanic.