From Bulbapedia, the community-driven Pokémon encyclopedia.

The Charicific Valley Natural Reserve, best known as the Charicific Valley (Japanese: リザフィックバレー Lizafic Valley) is a location in the Pokémon anime.

Located just outside of Violet City, the Charicific Valley is a training ground for Charizard so they can become stronger. It is distinguishable by its rock formation in the shape of a giant Charizard.

The only known human who lives in the valley is Liza, who, along with her Charizard, Charla, protects the valley from danger.

Ash left his Charizard here to train when he was heading through the Johto region. Noticing how much his Pokémon enjoyed spending time in the valley, and noticing that it was one of the weaker Charizard there, Ash decided to leave it in the valley so it could train. Since Charizard has been in training, it has become one of the most powerful Pokémon in the series, learning many new attacks including DragonBreath, Overheat and Steel Wing. It has also formed a romance with Charla.

Anime-exclusive locations in Johto

Alto Mare | Arborville | Battle Park | Big Town | Bloomingvale | Bluefinland | Blue Moon Falls | Blue Lagoon | Blue Point Isle | Bonitaville | Catallia City | Charicific Valley Coastline Gym | Diglett Village | Dragon Holy Land | Eggseter | Florando | Greenfield | Happy Town | Inland City | Lake Lucid | Len Town | Marion City | Maroon Town | Megi City | Ogi City Ogi Isle | Onix Tunnel | Palm Hill | Pudgy Pidgey Isle | Red Rock Isle | Remoraid Lake | Remoraid Mountain | Rikishii Town | Sento Cherry Town | Scarlet City Sea Temple | Silver Rock Isle | Silver Town | Snowtop Mountain | Sunflora Lodge | Transit Town | Whitestone | Wobbuffet Village | Yellow Rock Isle