Dying from basically the same Xerneas-cancer that runs through Will (and the rest of the “Ringed) for most of the story due to an investigation gone wrong at one of Xerosic’s labs (funded by Lysander of course) some years prior to the story. Since she doesn’t show up for most of the main game anyways, I figured I might as well justify that and use that as a harrowing motivator for Lysander to ramp up his plans so he can save her. Personality-wise, she was gonna be a kuudere type (but more realistically emotionally stunted) who only got good at acting because she learned to imitate others from an early age so as not to come off as a freak. Lysandre, who also had to put on masks for the sake of his duties as the heir to his family’s reputation (and their mafia affiliations behind the scenes) understood her the best and both he and Sycamore accepted her for who she was so she was most comfortable being her true awkward self with them.

Eventually she and Lysandre were on track to get married but then the latter’s darker affiliations got in the way of that when she, while leading a raid on known fugitive scientist Xerosic’s labs, got over-exposed to the same blue crystals that were used to make Will and others compatible with mega-evolution and she’d been deathly ill ever since. Though she didn’t know his connection to Xerosic’s, Lysandre felt so wracked with guilt for being indirectly responsible that he broke things off and plunged himself headlong into trying to rectify this mistake. He and Diantha rarely speak in the years following though he does keep very close tabs on her worsening health.

By the time the main story concludes with Lysandre’s death, Will obtains Xerneas’s power and banishes the disease from existence but by then it had taken its toll on Diantha’s body. She still pulls herself together one last time to accept Will’s challenge for her title, outrightly refusing his offer to heal her and even mega-evolving her Gardevoir during the course of the battle because when all was said and done, she wanted to go out on her terms (since the illness pretty much sealed her to a life of everyone else making decisions for her and fucking things up in the process cough Lysandre cough).

Will wins by the skin of his teeth (and a well-placed woodhammer from Almonde), Diantha is satisfied, tells Will to apologize to Sycamore on her behalf, admits that in the end “I would have loved to perform as Mrs Fleur-de-Lis” and dies on the spot. (Her Gardevoir follows suit but only after a verbal middle finger to Will aka the walking reminder of the man who ruined her trainer’s life).

Very sordid ascension, you can see why Will left the position after making sure to thoroughly clean out the remnants of Xerosic’s and Lysandre’s forces within Kalos.