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The most underrated rival character in all of Pokémon is Marnie, and im not joking.
>Not your childhood friend Barry, May, Hop, Hugh, Cheren or Bianca
>or related to the local expert in the starting area like Blue, Hau, May, Hop,
>Not super friendly like Hau, May, Bianca, or Hop, Trevor, Tierno, Shauna or Serena
>Not a twist villain or anything to do with them (Yell are just Spikemuth hooligans) like Silver, N, Bede or Gladion
She isn't any of those things.
She's literally just a strong trainer who happens to bump into you on your adventure, matches close to you in strength, and decides to surpass your pace and strength in time for your grand rematch in the final tournament. She's not overly intrusive, only battling you approximately 4 times across the adventure (balanced out greatly by Hop and Bede of course) so she never feels like she's tagging along with you, she's on her own path.
She is, without irony, the purest definition of a rival character.