Ok, I'm going to be the one who cheat and say Verity.
Really, she was a nice character and the whole thing of her being Cynthia's daughter had a lot of potential. I really wanted to see more of it. Sorrel too, though my ideal group would be quite crazy: Ash, Verity and Sawyer lol
Only logic I have is that she was a social outcast and had no friends till she met that reckless loser boy at the camp who so conveniently had a rag.
Ok, that might stray a bit on fanfic territory but...
The way I see, Serena always seemed like a girl who live a quielty, reserve life, in a "Classic Disney-Princess-y" way. Didn't have much friends around because her mom initially kept moving duo her Rhyhorn racing and then going to a smaller place to just make all the noise and attention go away.
Then summer camp came. Grace possibly put Serena there because she thought she'd enjoy some fun and could make some friends. Since Serena didn't want go, she couldn't make much friends (maybe not many girls around there?).
Afterwards, she got lost on forest. The girl who was mostly spoiled, pampered by her mother, who are used to a quiet and reserve life is now lost on wild. She is scared, a wild pokémon just jumped on her, she injuried her leg (or knee, or ankle, I really don't remember but it was on her leg lol) and she is all alone.
Then it came... A human face! Someone with a gentleman-ish attitude, speaking to her with a soft voice, conforting her, helping her deal with pain and just motivating her.
To add to that, said boy give her his hand, there was an accident hug where - considering how anime does stuff - Serena could feel the almost-Saintly kindness that boy transmited and then he just helped lead her to safety.
No wonder a girl like Serena would think she just found Prince Charming on Oak's Summer Camp of all places.
Flashforward to XY episode 1, Serena probably moving on, considering everything a vain dream or delusion. She was sick of her current quiet life, having to settle with just being Grace, the Rhyhorn Racer 2.0. Then, while watching TV out of boredom, she sees all the madness on Prism Tower. A rampaging pseudo-legendary just wreacking havok and a random trainer being the only one who seem to be doing something about it. So while watching all that drama until it reaches the climax: Garchomp is safe but a cute Pikachu is falling for his dead! But, wait! That trainer just jumped to sacrifice his life and save it! What a hero!
We reach the end where there's finally a close on that trainer face, his name is mentioned (or not, I don't remember) and then Serena sees.
It's that guy! From her childhood! Prince Charming came back to rescue her once again! She can't just skip Rhyhorn Racing! She can be free! And she can see him again!
A burst of feelings, Serena remembers everything. Ash's kindness. Tenderness. Attention. Thus she decided she has to see him again. I bet that had Clemont and Bonnie not be around, she'd attempt a confession. Though it'd fail, only the final episode Serena - after everything she's experience and learned - would be able to do that.
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Ok, that went longer and more melodramatic than I thought but long story short, I'd be more surprised if a girl like Serena not falling for Ash considering how she met him and how she got to see him again after so many years.
True although it would be interesting to see one of Ash's form rivals return as a traveling companion.
Sawyer.
I used to want Gary but nowadays, if I have to choose one former rival to be promoted to main character is Sawyer.
Gee, he should have never been a rival in the first place. Rather he should have been the companion. Maybe just have him replace Bonnie and Clemont after Ash won his fifth badge.
Serena coming back for the Galar region would be nice.
Never gonna happen, but delusion never stopped fans before
*Insert "Satoshi-sempai" joke here*
Anyway, I'd love to see Serena coming back. I feel like there's a lot more to be done with her character.