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Diantha is more of a celebrity and status symbol than an active Champion. You find her hanging out in the cafes or at the Battle Chateau every so often, but she doesn't have a major role to play in the plot of the game. She even says that she only trains in her down time, which along with her absence from the plot makes her feel passive or easy to ignore. It doesn't help that XY is really unbalanced in the player's favor, because even younger players might steamroll her party with Lucario or something. I think she's nice and inoffensive, but she doesn't leave the biggest impact for that reason.
Alder was a mentor to young trainers, periodically appears throughout the story to pass off wisdom to the characters. His absence from the Pokemon Leageu is also explained by his backstory, losing his old Pokemon and slipping into depression. Only stepping out of retirement to fight Team Plasma (even though he was out of his depth). That's more characterization and depth than EVERY other Champion in this series, and it's a genuine tragedy that the playerbase is more enamored with Steven or whoever.
