Clair and Morty for sure. Seeing a super-cool female gym leader who trained dragons (which I was really passionate about growing up, LOL) was so important to me. Like, it was honestly and truly a gamechanger, and the fact that she was so hard for me to beat (I always picked Cyndaquil) just made it even better, because whoa, here was a super cool, usually-miffed-at-something, hardline gym leader who trained the coolest and most difficult type (to train and evolve) of all and also straight-up refused to even give you your badge. It was mindblowing for me. Pretty sure I would have counted her as my role model if anyone had asked me back then, haha.
Morty was kind of different. I was one of the legions with a crush on him, but this was also the first time the Ghost-type was a focused-on thing even though the Gastly line had existed in RGBY, too. I'm also a sucker for legends, especially in Johto, so I loved Ecruteak already, and the kinda-weird guy who's training to be able to see the legendary phoenix (which was also interesting to note, that a Ghost-type specialist wanted to be able to see a legendary that can revive the dead) was really interesting.