Raine - Staraptor
Liam - Sceptile
Nadia - Teddiursa
Cierra - Pancham
Magnie - Cufant/Scizor
Bereth - Absol/Hydriegon
Eileen - Mutian Yanmega
Jamaya - Hullaballoon
Zifu & Zorona - Urshifu

For Ranae, I initially would've said Skitty, though now it's a little up in the air for her as to her "team mascot"! Emmie's would be either Toxicroak or Toxapex!

I'd say Victini. Even if he's not actually part of Briar's team, I think I consider him her team's mascot. Represents victory, and I call Briar's team the 'victory team' (:

Renahv: either shiny haxorus or marshadow I think.

Sableye for Cecils team absolutely! Its super hard not to just rename all Sableye into Hopper for me personally LMAO

For Norra, it's Chipper 100%

For Yorra, easily Buttons (though if some think Mugen then it wouldn't be too far off)

For Emille it's Dog, his Brionne, via sheer force of will and personal marketing thanks to how savvy she is with his instagram account LOL

Avian: Burnt out failure of a protagonist has his Charizard as his mascot. Regular 'zard, not shiny - he isn't that special.

Stahl: Corporate dad has his massive punk Machamp for maximum contrast.

Arryn: Lucario all day every day, her only companion and relic from her lost past in Hisui.

Mary's is probably her Banette, Quin. Since her name is just a pun for "Marionette" it seems appropriate.
Jin's might be either her Audino or Lucario?
And Celeste's is absolutely her Clefairy.

Jasmine: Her whimsicott, Dumb, Cute and can be used as a pillow.

Andres: His ralts, which he caught after starting to work on the academy, He actually takes pics of him when he's doing cute things

Ace: for public appeal he uses his Blacephalon, he uses it for special Events on his casino and public perfomances since its such a Rare beast

Kingston: aegislash QWQ his story wouldn't be the same without Than
Ruth: Dragonair bc I've drawn them together the most bc worms are easy to draw lol
Mira: unsure
Sadie: wooloo!!

Definitely Mauve the 'Espeon'

Though she likes to joke sometimes that Odd is more a mascot than her between the two-//

Easily Estella's Cinderace, Nike

Fausto’s Excadrill Tekken, it’s the physical embodiment of his whole character; small guy, big trouble

His shiny Eevee named Cloud. Usually he'll be excited to run off to the battlefield to fight and do a good af job doing it, however when Tyler asks to sit on the bench, he'll happily cheer him on.

Plus as Tyler's shoulder Pokemon, he represents his teammates with his sky pattern scarf! Plus he has a bad habit of running off and jumping up on strangers, so when people think of Tyler's team, they picture Cloud