As a Caterpie, he has a very happy but shy attitude; he was shown to suffer from low self-esteem if he didn't make a good first impression; he was very hurt when Misty didn't take to him. He is also very brave, squaring up against a Pidgeotto and then both of Team Rocket's Pokémon single-handedly.
As Metapod, he was shown to have become very laid-back, but remained extremely loyal to his trainer; this was shown when he jumped in front of a Beedrill to save Ash.
As Butterfree, his personality was similar to that of Caterpie, retaining his bravery and positive attitude. But as seen in Bye-Bye Butterfree, his self-esteem still suffered when he didn't make a good first impression.
Butterfree was caught as a Caterpie and evolved to a Metapod. Metapod then evolved when defending Ash from a flock of Beedrill. He caught it only by throwing out a Poké Ball. He once traded his Butterfree for a Raticate, but he missed his Butterfree so much, so he traded back. It seems to be a male Butterfree and was released to start a family with its mate, a pink Butterfree. Before winning the heart of its mate, Butterfree saved a bunch of wild Butterfree from Team Rocket (this caused the pink Butterfree to start having feelings for him).
- Butterfree marked many firsts for Ash's Pokémon: It was the first Pokémon Ash captured, the first to evolve, the first to reach his final form, the first to be traded away (Ash still got him back), the first to have a confirmed gender, the first to be released and the first dual type Pokémon Ash ever caught.
- As a Caterpie, Rikako Aikawa voiced it in both the Japanese and English versions of the anime. When it evolved into a Metapod, though, Aikawa continued voicing it in the Japanese series while Eric Stuart who played Brock in the original 4-Kids series voiced Metapod. Upon evolving into Butterfree, Aikawa began playing the role in both versions once again.
- Butterfree holds the record out of all of Ash's Pokémon for taking the shortest time to fully evolve, evolving from Metapod only one episode after it evolved from Caterpie. To compare, Ash's Charizard took three episodes to fully evolve, and his Goodra took four.
At Oak's Lab
At Kukui's house
At Cerise Laboratory