Any characters or professors from past gens that really stuck out to you as characters? I gotta say, I think GF put the most time into developing their Unova characters such as N, Skyla, Drayden, etc as they actually felt important to the plot and character with their deeper involvement and assistance. This region really stuck out to me because of this. Cynthia was also a really cool and impressionable champion for me because I was so young. She was... so cool!
For me, the characters that stuck the most to me were Cynthia and Hugh, the rival from B2W2. Both of them felt like actual people to me, which doesn't happen often with Pokémon characters, to be honest. Cynthia was simply very cool, and Hugh's backstory was very touching for me... They'll always be characters I'll remember
There were a lot, actually, but memorable characters include N, Silver, Eusine, Lance, Clair and many other Gym Leaders like Cilan, Chilli and Cress. I believe N had a powerful character that enhanced the deepness and movement of the plot. So much effort was put into creating these characters and all Pokémon characters, to be honest.
I like the villains in Pokémon more than anything else in the world. So Cyrus, Maxie and Colress really made my 3rd, 4th and 5th gen experience memorable. I know in my Rank the Villains topic I said that Ghetsis was a terrible character but he stuck out to me to because he was the only villain so far that tried to straight up kill the main character without the world being destroyed first, so Cyrus and Lysandre don't count.
One particular character that stands out to me was Steven Stone from Emerald specifically. I never played either Ruby or Sapphire (no money T.T), but I did manage to lay my hands on Emerald. Steven was weird, he kept appearing, but had little to do with the league or as far as I knew the plot. He just seemed like the normal business heir. Then Meteor Falls. I never expected it coming. He just wiped my team. Oh man, that is one memory I will never forget. Others that stood out to me are Whitney (that Miltank), Erika, Winona, Wallace, Morty, Eusine, and most of the characters in the Fifth Gen. Ah, fifth gen was one well-developed game, with seemingly real characters.