
Chikorita, Tepig & Totodile
At the beginning: You pick one of them as you begin the game.
Wild Zone 20: In the post-game, Wild Zone 20 (in the middle of the city) unlocks. You can encounter all three Legends: Z-A starters from there.

Bulbasaur, Charmander & Squirtle
Side Mission 022 (A Call from Mable): In this side mission, you pick one of the Kanto starters to keep. To unlock the mission, you need to defeat Rintaro in Main Mission 08. After healing in a Pokemon Centre, you'll receive the call from Mable to collect your starter.
Wild Zone 20: All three Kanto starters can also be encountered in Wild Zone 20, unlocked in the post-game.

Chespin, Fennekin & Froakie
Side Mission 007 (A Feisty Chespin): Complete Main Mission 05. Then, go to Academie Etoile and talk to Axi, who is standing near a taxi. Defeat Chespin three times, and you'll be able to add it to your team.
Wild Zone 17: Complete Main Mission 24. This unlocks Wild Zone 17, where you can encounter wild Chespin.
Side Mission 008 (Get Well, Fennekin): Complete Main Mission 05. Then, go to Magenta District and talk to Branche, who is standing in the grass beside a building. Accept the mission, which involves you collecting a Revitalizing Twig to heal Fennekin. Bring it back to Branche to get Fennekin.
Wild Zone 18: Complete Main Mission 30. This unlocks Wild Zone 18, where you can encounter wild Fennekin.
Side Mission 009 (A Challenge from Froakie): Complete Main Mission 05. Then, go to Jaune Sector 8 and talk to Surv, who is standing under scaffolding nearby the Pokemon Centre. Accept the mission, which involves climbing three courses made of scaffolding under time pressure, before you're invited to take Froakie.
Wild Zone 16: Complete Main Mission 24. This unlocks Wild Zone 16, where you can encounter wild Froakie.
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