It was raised in the Hidden Village by Melanie, who cared for sick, injured, and abandoned Pokémon. Bulbasaur acted as the village's bodyguard, but after Ash helped defend the Hidden Village from an attack by Team Rocket, Melanie encouraged it to join Ash, explaining that its growth was being stunted by staying too long in the village. Although Bulbasaur initially appeared to distrust Ash, it agreed to join him. In Island of Giant Pokémon, it alone assumed that Ash had abandoned it and the others. Its views on Trainers were presumably colored by all the abandoned Pokémon in the Hidden Village. Over time Bulbasaur grew to trust its new Trainer.
Bulbasaur is extremely stubborn, surly, and eager to battle. It had the opportunity to evolve after battling a powerful Rhyhorn with a very strong Take Down attack in Bulbasaur's Mysterious Garden, but refused at a mass of Ivysaur as well as a Venusaur because it doesn't want to evolve at Bulbasaur evolutionary festival. This allowed it to learn the powerful SolarBeam technique earlier than if it had evolved. It has the heart of a fighter and it never gives up in a battle. It has the determination to keep fighting even if the odds are against it, defeating such varied opponents as an Exeggutor, a Venonat, a Beedrill and a Scyther despite the type disadvantages. On one occasion Bulbasaur, along with Pikachu, was briefly lent to Misty to allow her to compete in the Princess Day Festival with a more balanced team, Bulbasaur single-handedly winning her first match by defeating a Kingler, Pinsir, Cubone and Raticate on its own. During a visit to a garden full of exotic plants run by Florinda Showers, Bulbasaur developed a certain affection for her Gloom, but nothing apparently came of it.
Bulbasaur is the only member of Ash's original team, save for Pikachu, to remain in his care. It stayed with him until midway in the Johto adventures, when it was sent back to Professor Oak's to keep the Pokémon there from quarreling with each other. Bulbasaur later returned in Hokey Poké Balls! to meet May's Bulbasaur, and again alongside Charizard and Squirtle to battle Brandon at the Battle Pyramid, where it defeated Brandon's Dusclops and Solrock (defeating Dusclops after its impressive defeat of Charizard, and taking Solrock down with it after a dual SolarBeam).
Bulbasaur is frequently viewed as the responsible leader of Ash's Pokémon, especially due to its actions at Professor Oak's lab. It is also a Pokémon Ash values more than most of his other Pokémon, possibly because he always wanted to catch one, as explained in Bulbasaur and the Hidden Village. It also formed a bond with Pikachu and Squirtle while they fought together while part of Ash's team. Oddly, when Squirtle was leaving Bulbasaur looked away while shaking hands. Also at the start he disliked trainers as shown in island of the Giant Pokémon where he said, "Maybe Ash has forgotten about us,"
It reappears in DP182 in Professor Oak's Laboratory, where Heracross is sucking sap from its bulb, much to Bulbasaur's annoyance. It used Vine Whip to shove Heracross away.
It will reappear along with all of Ash's Pokemon in an upcoming episode of Best Wishes Season 2.
Bulbasaur was the first Grass-Type pokemon that Ash has owned.
He is the second Pokémon Ash owns that chose not to evolve, the first being Pikachu, but he is the first Pokemon that Ash caught that chose not to evolve.
Bulbasaur is possibly the de-facto leader of Ash's Pokémon that are at Prof. Oak's lab.
Bulbasaur is the only one of Ash's original six pokemon, (other than Pikachu since Pikachu is always with Ash,) that he still has. Although he still owns Charizard and Squirtle, and can summon them whenever he chooses, neither of them live at Professor Oak's lab, unlike Bulbasaur.
Despite being the type, Bulbasaur does not know a Poison-type move.
Bulbasaur is Ash's first Pokemon which he caught and can be called upon