Leaf is a Pokémon Trainer from Kanto and is the female protagonist of FireRed and LeafGreen. Like
Lyra from HeartGold and SoulSilver, she is designed based off another character, green-game, who is from promotional material Ken Sugimori made when Red and Green just came out.
Game canon
In FireRed and LeafGreen, she sets off on a journey with her rival blue-oak, and takes on Gym Leaders, takes down Team Rocket, and becomes the Indigo League Champion while filling out Professor Oak's Pokédex. In canon however, she is just a good friend of Red_Anime and Blue.
She makes an appearance in Pokémon Masters EX. Her partner is Eevee or Venusaur.
Spinoff games
Leaf can appear in Pokémon Colosseum and XD: Gale of Darkness if you fight against a player with FireRed and LeafGreen.
Leaf makes a cameo along with Red_Anime in the intro of Pokémon Battle Revolution.
Leaf makes an appearance in Super Smash Bros. Ultimate, simply named (the female) "Pokémon Trainer". In this, she uses a Squirtle, Ivysaur, and Charizard.