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From what little we know
Clemont and Bonnie left the city because the gym had to be closed due to renovations
Synacmore left Kalos with Trevor to do field on research in another region
Looker never returned to Kalos
The team flare guy is still in prison along with the rest of Team flare who weren't useful enough to get plea deals like Marbel who, as part of the agreement cannot leave the city
Tieno is tearing up the dance floor in Unova
Calum,Serena and Shaua are completely unmentioned personally only as a collective by Lsyandre, Marbel and AZ
Diantha is still an actress, apparently working on a new movie
Malva is gone from the elite 4 and Kalos
the knight guy on the elite four quit to focus more on his cooking
Lida is implied to be Tierno's little sister
This game seems to be taking place shortly after Gen 7 and 8 as multiple references to Alola are made including Zygarde itself having come from there and Harmony/Paxton being natives on vacation in Kalos
That's about it as far as "Where are they now"
This game also implies Volo is apparently still alive in the present leading to theories he probably ended up caught up in one of those time distortions you can find in Hisui to catch modern pokemon like the Porygon line
Unbeatable Leon was defeated by Victor/Gloria already
The La Primeria title is referenced along with a "That battle happy young girl in Paldea" referencing Nemona from some rich guy NPC
sadly there are little references to events of Kanto,Johto, Hoenn and Shinoh or even Unova
