Team Rocket has been a staple of the Pokémon series since its anime debut in the '90s. Over time, the show has revealed more about the goofy trio and what led them to become members of the evil organization, despite being so good-natured at heart. However, one member's story gets more and more tragic the deeper you go.
Jessie's mother Miyamoto was a high-ranking member of Team Rocket and was the first person to capture an audio recording of Mew during a mission to South America. She was sent to try and capture Mew by Madame Boss, meaning she had to leave a young Jessie behind. During this mission, she was caught in an avalanche and is presumed to have died. Jessie, only being around five, was then sent to live in a foster home.
Things weren't any easier for Jessie in this new home. There was very little money, and Jessie mentions that there were days where the only thing there was to eat was snow. Despite her circumstances, Jessie looks back on these times fondly. As she grew, she had dreams of making it big and becoming famous. Sometime during her childhood, she had a crush named Astin who wanted her to leave with him and become a Pokémon Coordinator, but Jessie stayed behind because she and two friends were training to become pop idols. While her two friends passed their auditions, Jessie didn't.
After she came of age, Jessie left home to study at the Pokémon Nursing School, where she trained alongside a class of Chansey -- which was because she didn't qualify to study at a school for humans. She befriended one of the Chansey there and showed talent with some of the skills needed to be a nurse. However, being a human, she was unable to use Sing like her classmates and was unable to graduate.
The timeline of Jessie's life after Nursing School is a bit vague. At some point she was a model, she joined a biker gang, she was a weather girl and she supposedly had a brief stint as a ninja. How she met James has been retconned a few times, with them meeting at Pokémon Tech, meeting in a biker gang or meeting after joining Team Rocket. Despite how they met, they've been together since and it's definitely for the better.
Without Meowth and James, Jessie's life most likely would have turned out much differently. While she still would have been a part of Team Rocket, she probably would have worked as a lone operative and, and given how well her plans have worked out in the past, likely not a very good one. These failures, piled on to those of her previous ventures, most likely would have taken a large toll on her mental health and she wouldn't be as happy as she is now. But luckily for her, she has James and Meowth to keep her spirits up and they can continue to fail together.