Cassidy is a member of the criminal organization Team Rocket and a recurring antagonist in the spin-off series Pokémon Chronicles, where she serves as a counterpart and rival to the more famous Team Rocket trio of Jessie, James, and Meowth. As an official agent of Team Rocket, she generally operates with a higher degree of competence and resources than her bumbling rivals, though her plans are almost always foiled by the end of an episode, resulting in her and her partner being sent blasting off.

Cassidy hails from the Kanto region and was a recruit at the Team Rocket Academy, where she trained to become a full-fledged agent. It was during this period that she first met her future rival, Jessie, and her long-term partner, Butch. Cassidy, alongside Butch and his Raticate, were the top-performing team of trainees in the Blue Team, setting a standard of excellence that she feels Jessie and James have consistently failed to meet. This backstory establishes that her disdain for Jessie is not simply professional but has deep personal roots stemming from their shared history at the academy.

Cassidy's personality is defined by a cold, ruthless, and highly professional demeanor, at least compared to the theatrical and often incompetent Jessie and James. She is proud of her rank and her standing within Team Rocket, and she views the trio as worthless, low-ranking failures who bring shame to the organization. Unlike Jessie, who is prone to dramatic and violent outbursts, Cassidy is much better at keeping her cool and approaching missions with a strategic, if ultimately flawed, plan. She is extremely loyal to the Team Rocket boss, Giovanni, and to the scientist Dr. Namba, whose advanced technology she frequently uses in her schemes. Despite her professional pride, Cassidy is not immune to failure. Like her rivals, her plans to steal Pokémon often collapse in humiliation, which takes a visible toll on her pride and leaves her just as distraught as Jessie and James when they are defeated.

Cassidy's primary motivation is to advance her career within Team Rocket and prove her superiority over her rival, Jessie. She believes that she and Butch are the organization's best agents and that Giovanni prefers them, a point she often taunts Jessie about by mentioning that the boss sent the trio to far-off regions like Hoenn to get rid of them. Her personal vendetta against Jessie is a powerful driving force. Flashbacks reveal that the two were once friends, but their relationship soured due to Cassidy's jealousy over Jessie's beauty and her own aggressive, competitive nature. This rivalry is more personal than professional, and Cassidy will sometimes go out of her way to humiliate Jessie, such as temporarily recruiting James onto her team just to annoy her former friend.

In the story, particularly within Pokémon Chronicles, Cassidy takes on the role of a recurring antagonist. She and Butch are often depicted working directly for Dr. Namba on his secret "R project," which involves capturing rare and powerful Pokémon for the organization's nefarious purposes. Their missions frequently bring them into conflict with the protagonists, as well as their arch-rivals, Jessie and James. Her role is to serve as a foil to the main Team Rocket trio, showing that other, more capable agents exist within the organization, even if they are ultimately just as unsuccessful in their endeavors.

The most significant relationship in Cassidy's life is her partnership with Butch. They have worked together since their academy days and function as a well-coordinated team. A running gag in the franchise is that almost no one can correctly say Butch's name, instead calling him names like "Biff," "Bill," or "Bob." Cassidy is ironically one of the few people who can get his name right, though she has been known to get it wrong on occasion, much to his exasperation. Her other key relationship is her bitter rivalry with Jessie. She holds Jessie in contempt, considering her a loser who failed upwards due to luck, while Cassidy believes she has earned her place through skill and loyalty. She also shares a professional, loyal relationship with Giovanni and Dr. Namba, though this is less personal than her dynamic with Butch or her hatred for Jessie.

Cassidy's character does undergo a notable development, especially in later series beyond Pokémon Chronicles. After years of service, she and Butch eventually leave Team Rocket entirely. They abandon their life of crime to open a small café in a snowy village, symbolizing a complete departure from their past ambitions. In this peaceful new life, Cassidy appeared to have finally ended her long-standing feud with Jessie, even offering food and shelter to her former rival and her companions. This change represents a significant shift from a cold, ambitious agent to a person capable of letting go of past grudges.

Regarding notable abilities, Cassidy is a skilled and intelligent strategist who prefers to use elaborate disguises and advanced machinery rather than relying solely on Pokémon battles. She has posed as a breeding center owner and a fortune teller to lure in victims and has used technology like mind-controlling Drowzee, rage-inducing crowns invented by Dr. Namba, and giant robots to capture Pokémon. As a Trainer, she is competent and has been seen using Pokémon such as Raticate and Houndour in battle. However, in a direct fight, she and her Pokémon are generally no match for the combined efforts of Ash, his friends, and even the chaotic interference of Jessie and James. Despite her claims of superiority, her track record of failure is largely similar to the trio she so openly mocks.