For me it's Bianca, since she played such a part in both sets of games, and we got to see her grow up through the course of them. She started out indecisive and insecure and scatterbrained, then throughout the first games she started to learn to assert herself and to figure out what she wanted to do with her life, so that by the later games, she essentially assumed the role the "professor tree" plays in the games, as the adult who gives us our start. She, to me, is the most notable common thread between the games, and it was nice to see her grow to be decisive and secure and... still a bit scatterbrained. It provided a good sense of the passage of time there AND of continuity. And it was nice that she still had her Munna with her (now a Musharna), and that much nicer that the two of them together were really pretty impressive in Reversal Mountain. They came a long way together, and that was very satisfying to see.