The Kanto first partner Pokémon are the Pokémon chosen by Trainers at the start of their journey in Kanto. These are traditionally given out by Professor Oak and are to be used to battle the first wild Pokémon that the Trainer encounters.
In Pokémon Red, Green, Blue, FireRed, and LeafGreen, Professor Oak will bring Trainers to his lab to give them one of the Kanto first partner Pokémon to begin their journey before offering the player's rival the same opportunity. They will always select the type-advantageous first partner Pokémon, and the remaining Pokémon will sit in its Poké Ball on Oak's desk for the rest of the game.
In Pokémon FireRed and LeafGreen, the choice of first partner Pokémon also determines which of the Legendary beasts will roam the Kanto region after defeating Team Rocket in the Sevii Islands—if the player chooses Bulbasaur, Entei will appear, Suicune will appear to those who chose Charmander, and Raikou will appear to those that went with Squirtle.
Professor Oak offers one of this trio of Pokémon to the player in Pokémon HeartGold and SoulSilver after the player has defeated Red, making reference to the fact that he gives them out as first partner Pokémon in Kanto-based games, suggesting the player will be able to make it to the next town with one of these Pokémon by their side.
In The Isle of Armor, Honey will let the player choose either Bulbasaur or Squirtle after completing the Master Dojo's first trial. Once fully evolved, these Pokémon will be capable of Gigantamaxing. Honey will keep the Pokémon not chosen by the player, and use it on her team.
In The Crown Tundra, Ivysaur, Charmeleon, and Wartortle can be found in Dynamax Adventures.
A poster on Ash Ketchum's bedroom wall, depicting the Kanto first partner Pokémon
Pokémon the Series
Over the course of Pokémon the Series, Professor Oak has given away three of the Kanto first partner Pokémon, two of which were to new Trainers. Gary Oak and May both received a Squirtle, and Gilbert began his journey with a Bulbasaur. Despite not having known to handout a Charmander, Professor Oak has been shown with all three first partner Pokémon in his possession. Professor Oak has also given out a Pikachu to Ash when he was late to his pick up his first partner Pokémon and had missed out on receiving one of the usual three choices.
It is revealed in Pokémon the Series: XY that Professor Sycamore gave Shauna, Tierno and Trevor a Bulbasaur, Squirtle and Charmander respectively, and in the Mega Evolution Specials that Alain was given a Charmander as well.
I Choose You!, a movie that started a separate continuity from the main Pokémon the Series continuity, focused on the developing friendship between Ash and the Pikachu he received from Professor Oak. In this continuity, Gary also started with a Squirtle, while the Oak's Bulbasaur and Charmander were revealed to have been picked by alternate versions of Marina and Tierno, respectively.
Pokémon Evolutions
In Pokémon Evolutions, although Professor Oak doesn't hand out any of the Kanto first partner Pokémon in this series, he is shown to have them in his possession.
In the manga
Pokémon Adventures
In Pokémon Adventures, Professor Oak is shown to give Pokémon alongside Pokedexes for Trainers he thinks are especially talented. Most characters are shown to already own another Pokémon before receiving their partner from Professor Oak. Gifted Pokémon include Charmander to his grandson Blue and Bulbasaur to Red. Professor Oak also possessed a Squirtle that was stolen by Green.
Professor Sycamore was also shown to be in possession of the Kanto first partner Pokémon and while Squirtle and Bulbasaur remained under his care, he Charmander to X which he nicknamed Salamè.
Pokémon Pocket Monsters
In Pokémon Pocket Monsters, Professor Oak is depicted distributing the choice of one out of three first partner Pokémon - Bulbasaur, Charmander or Squirtle - to Pokémon Trainers as they begin their journeys, much like in other media. Green chooses Charmander as his first partner Pokémon, and while Red was offered the choice as well, he decided to keep a Clefairy that he met on his way to Professor Oak's lab as his first partner instead.
The Kanto first partner Pokémon are the first trio to receive Mega Evolutions.
The Kanto first partner Pokémon can be obtained in the most games of all first partner Pokémon, as they have appeared in 20 core series games in Japan (19 internationally).