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It might be Elm. He’s kind of squirrelly and doesn’t stand out in Johto, when Oak is the big guy on the radio giving you tips about the game world.
If not him then Juniper. I genuinely remember little about her presence in the game apart from her dad.
Birch is May’s dad, and has that funny opening when he gets chased around by a puppy. ORAS built on that gag with his wife chasing him around in the postgame, which was a cute gag. He’s a funny, family man.
Rowan actually gets involved in the Team Galactic scuffle for a bit, when the Lakes get attacked and he leaves the lab to help.
Sycamore is a big socialite. He sort of knows everybody? He apprenticed under Rowan, studied with Lysandre, became friends with Diantha, and organizes your Championship parade in the city. The guy gets around, and had tons of fangirls when XY launched.
Kukui is even more outgoing, with his wife an obscure hobbies. Being the final boss was a nice touch, since he toured you around the map.
I skipped SwSh so I can’t speak on them.