Ash's Bulbasaur (Master)

Ash's Venusaur was the third Pokémon caught by Ash Ketchum in the Kanto region, and the first of the original starter Pokémon that Ash acquired.

Kanto
Ash first encountered Bulbasaur in the Hidden Village, trying to retrieve his Pikachu but Bulbasaur attacked Ash thinking he was stealing him. Ash cleared it up with him and Melanie, and Melanie suggested Ash keep him. Ash battled Bulbasaur, and captured him.

Bulbasaur was immediately used to stop Team Rocket from stealing the Pokémon in the Hidden Village, and protected Ash from a poison sting from Jessie's Ekans.

Bulbasaur was used to battle Misty's Starmie, Butterfree having weakened Starmie already. Bulbasaur struggled against Starmie, and refused to dodge attacks. Bulbasaur used razor leaf to finish Starmie off.

He also helped out on the SS Anne, helping fight off Team Rocket Grunts and helping save Ash, Serena, Misty, and Brock with his vines.

Bulbasaur later on battled Erika's Tangela but lost. He redeemed himself in a battle against Koga's sister, Janine's, Venonat and later on against Koga's Muk.

Bulbasaur encountered a mysterious garden where Bulbasaur and Ivysaur come to evolve, but Bulbasaur ended up not wanting to evolve, fearing Venusaur. Bulbasaur learned solarbeam in a battle against Venusaur, where Venusaur accepted Bulbasaur's decision.

Bulbasaur also battled Giovanni's Dugtrio, defeating it but falling to his Rhydon. Bulbasaur battled in the Indigo Plateau Conference, in one match against Jeanette's Beedrill and won using a clever tactic using leach seed.

Personality and characteristics
Bulbasaur is one of Ash's most mature Pokémon, and is viewed as the natural leader of Ash's team. Prior to being captured, Bulbasaur was the guardian of the Hidden Village and was extremely cautious of humans. He attacked Ash, thinking Ash abandoned Pikachu only to find out the truth. He began to trust Ash overtime, but still kept his stubborn attitude. Bulbasaur is friendlier to Pokémon though, and more protective due to his views of humans at his time in the Hidden Village.

Bulbasaur shows his more immature side when with Squirtle, often quarreling with each other and sometimes getting into physical fights. Despite that, the two have a strong friendship and play with each other the most.

Trivia

 * Bulbasaur is Ash's first dual-typed starter, and his only up until Alola.