I consider Olivia to be the dead ringer for 'sexiness', but if it is the personaliy that attracts, then my vote would be for Lillie. Her Z-Powered Form was the final decision. AND she went out to Kanto in the post-game events. Will she use Bulbasaur, Squirtle or Charmander? No one will know, even if a thousand fanfics were made titled 'Lillie in Kanto'. Unless someone did and we can read all about it in LillieInKanto.com. JK, broken server, so if anyone wanted a LillieInKanto.com, you can take it.
But she's kind, sympethetic, generous, lacking in malice and any other words that are thesaurus-worthy. She's got the one trait, but kindness beats malice, so she's the clear choice.