Tracey, Max and Brock.
Tracey gets title of most shafted character in history of pokemon series being there for just one measly season never having chance to be developed and make actual story around his character. We never learned anything about his past, he almost never battled, got just one pokemon capture and his career was generic and relatively simple ending up as prof. Oak assistant eventually. On good part reason is because Tracey was created for wrong reasons, being there only because of potential racial issues writers thought Brock appearance my have cause among western audience. Hence being reason why they never invested into character ditching him very fast.
Hard to top that really.
Max also almost never battled or competed in something but that can be excused considering he was too young to have pokemon. However aside from serving as source of comedy having fights with older sister, and serving as secondary navigator in group along with Brock, Max never really received much attention about himself nor writers developed concrete story around his character. Sure there were episodes which dealt with his knowledge and he played role in boding with several pokemon(mainly psychic), but most of time he was on sidelines with lack of pokemon and defined direction in which he could be taken really hurting his character.
Brock comes in this place. He was handled better than most male sidekicks until Cilan came, but compared to female protagonists he always received the least(including Iris and Misty who also got more).
He had his own goal of becoming top breeder, there was background and story present around character but problem is there was never out enough dedication toward it with development of character happening rarely and occasionally with big gaps being present between moments where he grew as character.
One of biggest problems responsible for this was because his goal of breeder was very passive with there being little you can do with something, which isn't about battling and entering competitions. Initial issues with breeding aspect is that you cant measure much progress of someone whose career is about taking care of pokemon and raising them to be in good shape.
While with more battle oriented quests writers have easier time in creating something for character, because it opens window of possibilities(like inclusion of rivals,competitions,training episodes etc).
As for girls May and Dawn obviously received more focus, got there own rivals and had defined directions in which their story could be taken.
So decision comes down to either Misty or Iris as next most shafted characters.
With Misty while she received some genuine and quality growth transforming from overly insecure and impulsive character in more mature and independent person improving her water trainer skills, main problem was that this things and Misty growth as trainer didn't happened on consistent basis, with big gaps existing between them.
Sure she often interacted with characters bringing comedy in cast and played sub-role in several episodes which weren't focused on her but character itself didn't had enough screen time to shine on his own and develop more. Writers left many things unexplored about Misty dreams,past,motivations etc leaving her story unresolved.
Iris on other hand has more defined goal, she has her own rival, there is put more development on her pokemon and growth of becoming dragon master. BUT as character she didn't changed much with Misty receiving more development in character sense with lot of struggle and hard work being present behind it.
In first 90 episodes Iris got slightly more focus and character centric episodes through, but since Misty stayed longer she obviously still leads. Unless Iris stays for extra generation, than that might change.
As for Cilan he receives pretty good focus for sidekick character receiving more than Brock did (when taking in account one saga and not whole run), but in development department he isn't handled very good either being done very little about connoisseur career which is still on large part left undefined lacking direction in which it could be taken. So his focus is handled so so, so far.
While Misty was around for a considerable amount of time, she was most there as nothing more than a third wheel on a guys day out or another kid for Brock to babysit. Though we were spoiled with her presence, she didn't really develop a whole lot. We see a lot of growth from May and Dawn because they had a clear goal to go after. They catch Pokemon, learn things, become better. Misty was already a gym leader and she just kinda decided to tag along. At least with Brock he had a new goal and everything he did seemed to help him in his quest. But this was before we really saw the necessity for characters besides Ash to have their own individual goals and pursue them.
Not sure when was last time you watched OS but that simply isn't true. Misty had her own story too, to become water pokemon master which was reason why she left gym to travel in first place, she had complex pf feeling less worthy compared to Daisy and others and wanted to explore world and learn as much as she can about water types.
She had more of role than just being "tag along" too, serving along with Brock as Ash coach and mentor(especially in earlier seasons)helping him to grow from greenhorn to somewhat competent trainer, she was there to provide moral support to cast and by acting like someone who would balance out Ash impulsiveness and naivety and Brock's obsession over girls with her spunky, and strong personality giving identity to group .
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She didn't developed much in goal sense that's true, but she grew a lot as character gradually growing in more mature and confident person with various sides of her personality being explored more showing how there was more to her compared to what was showed during debut . Which led to some great character development with Misty gaining sisters respect getting over complex of feeling less worthy, getting over Gyarados fear, growing as water trainer coming in light of more confident and independent person(top 8 in Whirl Cup, winning Seaking championship, Alto mare race etc kinda confirming that she improved) while still having charm.