>>44772552I would argue it started with Gen 4.
Characters started getting development, goals and personality beyond the basics like Barry changes a bit following his loss to Jupiter at Lake Acuity, or Silver growing from a butthurt bitch to just a bitch, and also his relationship with Giovanni. Gym leaders also started getting more character outside of just being Gym Leader too in Gen 4 too, like Crasher Wake and Volkner, seeing them outside of their gyms also helped there. Gen 2 had developed Kanto's gym leaders too, and HGSS built on top of that with more appearances outside of gyms and getting their phone numbers. Like seeing Falkner and Janine argue about their dads or taking photos with them.
But all of this was simply making the characters feel less flat, and leave an impression on you, without taking attention away from the Pokemon.
Gen 5 was a lot more character focused, with their own stories we see in the game.
Like Bianca dealing with her father and later realizing she didn't have the same goals as the MC or Cheren. Cheren figuring out why he wants to become stronger. N and the world he grew up with being turned upside down after meeting Hilbert/Hilda. Hugh and his sister's Purrloin. The B2W2 memory flashbacks also give more depth to Gym Leaders, who we see have arguably more important lives outside of being just a Gym Leader, like Clay, Burgh, or Elesa.
From here on, characters really start showing growth and character arcs.
Tbh, I'd also argue that human characters really start getting more popular compared to a good chunk of legacy characters outside of some exceptions.
