Kasumi, known internationally as Misty, is introduced as the fiery-tempered Gym Leader of Cerulean City, a position she inherits from her three older sisters, Daisy, Lily, and Violet. Feeling underestimated by her siblings, who often dismiss her as the "runt" of the family while they perform as the "Sensational Sisters," Kasumi leaves the Cerulean Gym to travel and prove herself as a trainer. Her journey alongside the aspiring Pokémon Master Ash Ketchum begins not by choice, but by circumstance: after Ash "borrows" and subsequently destroys her bicycle with Pikachu's electric attack, she decides to follow him until he repays the debt, a reason she eventually forgets as their friendship deepens.

At the outset of her travels, Kasumi is characterized by a short temper, a stubborn demeanor, and a distinct lack of patience, often serving as the voice of reason or the source of conflict within the group. She is notably terrified of Bug-type Pokémon, which often leads to humorous confrontations. Despite her initial abrasiveness, she consistently demonstrates a kind and sensible heart, particularly when it comes to the well-being of Pokémon. Her personality matures significantly over time; while she retains her assertive nature, she becomes a nurturing figure, most evident in her role as the primary caretaker of the mysterious Pokémon Togepi, which imprints on her as its mother after hatching from an egg Ash was carrying.

Her primary motivation is to become a world-class Water-type Pokémon Master, a dream she pursues despite her sisters' ridicule. She has a profound love for Water-type Pokémon and a deep admiration for the Elite Four ice-type trainer Lorelei, whom she looks up to as a role model. Her role in the story is that of a mentor and friendly rival to Ash, as well as a disciplinarian to their other companion, Brock, whom she frequently restrains by pulling his ear when he flirts with women. She travels with the group through the Kanto region, the Orange Islands, and the Johto region, honing her skills as a trainer through numerous battles and captures.

Key relationships define her journey. Her dynamic with Ash is a mix of rivalry and mutual respect, where she often critiques his impulsive style while simultaneously supporting his determination. Her relationship with her sisters evolves from one of inferiority to eventual reconciliation, as she learns to stand on her own merits before returning home to take full responsibility for the family gym. She also forms a close friendship with Sakura, a girl who shares a similar situation of having older sisters who treat her like a child.

Her development culminates at the end of the Johto League Silver Conference, where she finally receives a repaired bicycle from Ash, removing the last thread of obligation holding her to the group. Choosing to embrace her responsibilities, she returns to Cerulean City to become the full-time Gym Leader after her sisters leave for a world cruise. Although she steps away as a main character, she reappears in later series to support Ash, showcasing her growth into a confident and capable leader.

Kasumi is a competent battler specializing in aquatic creatures. Her team notably includes the psyduck that she "accidentally" caught, which has a habit of popping out of its Poké Ball and whose headache-induced psychic eruptions often turn the tide of battle. She also trains a goldeen, staryu, and starmie, which are staples of the Cerulean Gym, and she later captures a corsola and evolves her poliwhirl into a politoed. Although she eventually releases togetic after its evolution, her bond with the Pokémon that was once constantly by her side remains one of her most defining traits.