A quest for the missing 4 stars

[Tartempion]

I just recently finished maxing my 4 star characters dropped from the Sub Stories quests. However, I'm still missing a lot of 4 stars in the collection (and by a lot, I mean a lot). I know a few can be bought for Brave Battle medals, and some are exclusive to thematic lotteries (like Swimsuit or Halloween). However, I doubt it will fill the gaps at all.
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Where are the missing ones, then? Unobtainable?

[Beelzebozo]

you can find some locations here for missing characters but not all character color variation are given a location, but maybe you can find some on here you don't have.

https://www.bbs-simulator.com/

[Tartempion]

yes, I know this reference (very good website), but the ones I'm missing are not written. I'm starting to think that they were exclusive Point Event drops before the recent event changes and that they are just unobtainable for newer accounts...

[Amphibious Void]

ive been looking for sources on if its even possible to get the missing characters i havent obtained through main story and sub story, it seems like theyre just a bunch i missed out because i started playing 2 years ago...