(Liza and Charla as they appeared in the original series.)
>Liza is the caretaker of the Charicific Valley, which was made specifically as a natural paradise for wild Charizard.
>Charla belongs to Liza, who maintains the Valley. When Ash and his friends were going through the Valley, they had the opportunity to meet Charla. Charla is Liza's only Pokémon, and her greatest partner, seeing as she travels with her to places outside the valley. Charla was also the only non-wild Pokémon in the valley when Ash and his friends met her.
Liza and Charla appeared in the Best Wishes series in a flashback:
Source: Liza | Charla