Annabelle "Bella" Calaveras had her first brush with the supernatural at the age of 10.
She and her younger sister of 5 years, Nora, had gone into the woods to look for pokemon and/or gather berries. At some point the two got separated, and Nora ventured deeper into the woods to "take a closer look" at something. Bella tried to go in after her sister, but was jumped by wild pokemon. She was able to fight them off with her Rowlet, but was unable to find Nora, nor her beloved stuffed buneary, Nibbles.
A couple nights later, Bella awoke to a phantump and a banette knocking on her window. She went to investigate, and realized she was face to face with her dead sister (and Nibbles).
Looking for answers to what happened to her sister, where her own life should go from there, and the connections between ghost pokemon and the spirits of the dead and forgotten, Bella made it her life's mission to become a paranormal investigator and learn as much about ghosts as possible.
She may have taken things a bit far though, and became rather obsessed and bitter as she got older. She often loses sight of taking care of her sister and what she would have wanted, and just gets caught up in her own wants and possible revenge on whatever unknown (perhaps even unown?) force that took her. Since they're both paranormal investigators and specialize in opposing types, she's become kind of a rival figure to Lexa.
Eventually she moves to Dimia and becomes a Frontier Brain in the Perius League, earning the title of Tower Keeper.
Nora, meanwhile, has grown into a Trevenant (since phantump evolves via trade I assume she went back in the forest one day to be with her ghostly brethren but then wound up coming back) and can cast a mean curse.
Side note: Nora's 10th birthday was especially hard on Bella because even though she was technically there for it, she knew she'd never get to go on a pokemon journey the way they both wanted.