Musashi (Jessie) is one of the three members of Team Rocket. Her two other teammates are Kojiro (James) and Meowth.

Jessie made her debut in the anime in Pokémon Emergency. Alongside James and Meowth, they set their sights on the injured Pokémon in the Viridian Pokémon Center. Sending out their Ekans and Koffing from their Meowth balloon, they quickly overtook the Pokémon Center, which only had three people inside. Before they could steal any of the Pokémon, however, Ash's Pikachu, who teamed up with many other Pikachu in the Pokémon Center, used an electric attack, causing the entire Pokémon Center to explode. It was here that Team Rocket decided to capture Pikachu, deeming him as an exceptionally powerful Pokémon, and made it their goal to capture him.

Jessie's mother, Miyamoto, was a high-ranking Team Rocket agent. When Jessie was around the age of five, Miyamoto left on a mission to the Andes to record the voice of the elusive Pokémon Mew. However, after Miyamoto disappeared, Jessie was put into a foster home which had very little money. There were times when she had no choice but to eat snow because of a lack of food.

When she was old enough, she left home to become a Pokémon nurse. However, she was unable to go to a regular nursing school and went to the Pokémon Nurse School, which was intended for Chansey. She was quite skilled at things such as bandaging, and even showed a Chansey how to do it, which she quickly became good friends with. However, because she was not a Chansey, Jessie couldn't do things like use Sing to soothe Pokémon, instead falling asleep herself. Ultimately she failed to graduate, and on graduation day she simply packed a bag and left. As she was leaving, Chansey came up to her and offered her her nurses' hat, but Jessie refused to take it. Chansey then broke her egg-shaped pendant she got as proof of graduating in half and gave one half to Jessie so that they would have something to remember each other by. In the episode Ignorance is Blissey, Jessie was reunited with that same Chansey, which had since evolved into a Blissey.

It is assumed that she then enrolled in Pokémon Tech, a Trainers' school, where she met James, and subsequently failed and dropped out. After this, as revealed in The Bridge Bike Gang, Jessie and James joined a bicycle gang in Sunnytown, where Jessie was known for swinging a chain above her head as she rode her bike, acquiring the nicknames "Big Jess" and "Chainer Jessie."

Sometime before joining Team Rocket, and it is not known when, Jessie obtained a Contest Pass in the Hoenn region. Also during the years before Team Rocket, she had several boyfriends, all of whom left her and/or treated her badly. In Xatu the Future, she revealed that she had once worked at a TV station as the weather girl. In From Cradle to Save, it was revealed that she once was a ninja.

Due to conflicts in canon between Kanto and Johto episodes and the special episode Training Daze, what happened between Jessie and James after the bike gang is unknown. What is known, as shown in the special episode, is that they joined Team Rocket separately and were on initially unfriendly terms after being grouped with Meowth but they quickly made up and became good friends.

Starting in Hoenn, Jessie became a Pokémon Coordinator, entering every Contest that she could. She frequently made it to the battle round, but usually lost there. However, in Borrowing on Bad Faith!, she finally succeeded in winning an (unofficial) Contest by using Ash's Aipom, which she had saved earlier in the episode. And, in Team Shocker!, she won an official Contest with her Dustox at Solaceon Town. Later in Shield with a Twist!, she won a Ribbon with help of her Yanmega. She has Contest Passes to compete in the Hoenn, Kanto, and Sinnoh regions.

In the Kanto Contests, Jessie disguised herself as Jessebella (Japanese: ムサぴょん Musa-pyon) wearing a red dress and glasses. Eventually, Ash, Brock, Max and May found out her true identity in the last Contest.

Although Jessie entered mostly every Contest in Hoenn and Kanto, she lost in all of them, whether it was the first match of the Battle Rounds or the finals. However, in the Sinnoh region, she won three Ribbons herself using her Dustox, Yanmega and Seviper, and obtained a Ribbon after James won a Contest in her place, as well as receiving one from Princess Salvia after she lost to her. Jessie was then eligible to participate in the Sinnoh Grand Festival, and made it to the semi-finals, losing to Dawn. After her loss to Dawn, Jessie got furious as her dream of becoming Top Coordinator was destroyed, but regardless, she told Dawn that since she beat her, she wouldn't forgive her if Dawn didn't end up as Top Coordinator for as long as she lived, thus showing that Jessie was rooting for Dawn, despite the two being enemies. When Dawn then lost in the finals to Zoey, Jessie was furious and decided she was through with contests. She then discarded her Jessilina costume before storming away. None of the main characters ever learned that Jessie and Jessilina were the same person.

She's also used other aliases to disguise herself under, such as Jessilinda during her time at the Pokémon Summer Academy and Jessilynlyn while competing in the Pokémon Ping Pong Tournament. Against Marilyn in Battling a Cute Drama!, she used the alias Jessirilyn.

(Source: Bulbapedia)