Since we all know that Ash and Serena knew each other since they were kids, that also means she knew Gary, right? I thought about it for a bit and did some headcanons for the 2. Enjoy!
-After going to Hoenn, Serena decides to visit Ash in Kanto, only he's not there. So she goes to her other Kanto friend Gary.
-Guess what, he didn't forget her dammit.
-They talk about each of their adventures, Ash and other things.
-This all ends with them telling stupid stories about Ash and the dumb things he did.
-After that the 2 keep in contact with each other.
-And I'm talking full force, they text daily, and have weekendly video calls.
-It's usually just them bitching about life.
-Serena resumes her her adventure in Sinnoh.
-Gary is surprisingly good at love advice.
-Also, you can't tell me they aren't watching reality shows, like life with the Kardashians.
-Eventually Ash finds out about this and the 3 of them now talk together.
-Gary corrupted her in calling Ash, Ashy-boy.
-'Garry will pay for that' quote en quote
-Serena sometimes catches rare pokemon for Gary to study with his grandpa. She once caught a shiny skorupi that she's very proud of.
-Speaking of Gary's grandpa, Professor Oak is ecstatic to see her perform. Dropping every thing he does to watch her contests.
-And he's so into it, raging when she doesn't win and ranting to Gary.
-Her and Gary eventually visit Ash in Alola, and boy is it fun.
-She finds a cooking rival in Mallow and a gossip friend in Lillie.
-Gary might have developed a crush on a certain blonde with a vulpix...
-This gives Serena blackmail material.
-She convinces Ash to put Lillie on their little Kanto group chat.
-Now Gary and Serena can bond over another thing: having completely oblivious crushes.
-But she gives Lillie hints nonetheless.
-They eventually leave their separate ways, leaving Ash to win the league.
-After the Grand Festival, Serena goes to Kanto for vacation.
-During this time, her and Gary have gone shopping almost daily.
-He acted like a professional fashion designer the whole time.
-'Girl...That dress with that jacket? No no no, let Gary help you !'
-He's actually pretty good at those things, so she let him help.
-After she returned to Kalos she promised shell visit again after she becomes Kalos Queen.
-Ash and Gary surprise her and come to her Master Class Performance.
-She and Ash eventually become a couple (with a lot of pushing from Gary and Bonnie).
-After this Gary goes to Alola on a little ”research trip”.
