Bonnie had more notable moments and moments of character building than Serena had before the Showcases were introduced.
Are you purposely ignoring the moments where Serena shows her fragility and how spoilt she was (The road to Santalune and staying in the forest) her indecisiveness (When asked by Clemont why she left home), her attempts to find her goal and trying various activities (Baking, Grooming etc) being sad at not having a purpose on her journey (The time when she says 'It must be nice to have something you are so passionate about' or something along that line), Aria's introduction, the summer camp etc. Or are you just not remembering things correctly.
It's one thing to misremembered things, it's another to make blanket statements like someone is handled better without any reason when the someone has her first two focus episodes wasted thanks to the marketable Eevee. How is Koharu treated 'better', because she doesn't appear every episode ? If so, then every pokegirl is handled worse because none of them (except for Misty afyer the league) did anything during the leagues or many gym badges. Most of them they appeared in.
It's unfathomably stupid to expect every character to do something notable every episode. They aren't a single protagonist. They have to share screentime. If Koharu's merit lies in not appearing every episode, then you're just comparing apples and episodes by having Koharu already put in an advantageous situation of not having to put in work every episode. Then effectively ignoring Serena's character moments.
If Serena were to have all 47 episodes to herself, someone would have been complaining about other characters getting shafted. Completely missing the point that there are parts of a story where each characters gets focus. It was Clemont's time, but the fact remains that Serena still got major moments in a time where she wasn't even the major part.
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Extending to every character, it's stupid to ask them to do something every single episode and call it bad handling when they don't get focus in someone else's episode. Calling Ash useless because he does nothing in the Grand Festival is stupid. Calling May useless for not doing anything in Ash and Brandon's match is stupid. Calling Dawn useless because she wasn't bringing up her own troubles during Ash vs Paul is stupid. Are 'forefront' and 'limelight' such foreign words to you that some other character getting focus must mean that the character being discussed is entirely useless and worthless ?
If yes, then Ash is useless because he didn't snatch Braixen's pokeball from Serena and performed at the master class.