Ondine, known as Misty in English-language versions and Kasumi in the original Japanese, is a Pokémon Trainer and the Gym Leader of Cerulean City. She is the youngest of four sisters, Daisy, Violet, and Lily, who run the family's Water-type Gym. Growing up in the shadow of her older sisters, she chooses to leave home with the goal of becoming a world-class Water-type Pokémon Trainer rather than remain in the background. In the anime she enters the story when she accidentally fishes the young trainer Ash and his Pikachu out of a river while searching for Water-type Pokémon. Ash then borrows her bicycle to escape a flock of wild Spearow, and the bike is badly damaged by Pikachu's electric attack. Because of this, she insists on following Ash until he replaces the bicycle, and what begins as a demand for compensation turns into a long shared journey.

In terms of personality, Ondine is a stubborn tomboy with a fiery temper and little patience, especially in the early episodes. She is outspoken, proud, and quick to react, and she does not hesitate to scold others when they annoy her, often using physical comedic punishment to make her point. She is also sensitive about her appearance and dislikes being teased. Beneath this strong exterior, she is loyal, kind, and increasingly level-headed, and she often serves as the voice of reason within her group. Her sense of responsibility is strong; she is willing to risk her own safety to protect her Pokémon, and she carries family duty even when it is inconvenient. Two traits are especially notable. First, she has an intense fear of Bug-type Pokémon and screams or flees when confronted by them. Second, she has an exceptionally deep love for Water-type Pokémon, to the point of admiring even unusual species with enthusiasm.

Her main motivation is to become a world-class Water-type Pokémon Trainer, a goal she pursues with dedication despite her older sisters' initial mockery of her abilities. Her strategy in battle is an all-out attack approach built around Water-type Pokémon, and she is constantly seeking ways to improve her technique and expand her team. As a skilled swimmer she trains at the Seafoam Islands, and she also enjoys fishing, crafting her own custom lures. She holds other skilled Water-type trainers in high regard and aspires to reach their level. A sense of duty also guides her; when her sisters abandon the Gym, she accepts the responsibility of keeping it open and honoring the Gym Leader title.

In the story, Ondine is one of the main characters during the early seasons of the anime, traveling with Ash and Brock, and later with Tracey during the Orange Islands arc. Together they journey through the Kanto, Orange Islands, and Johto regions. Throughout this time she provides practical guidance, comedic moments, and support in battles against Team Rocket and other opponents. At the conclusion of the Johto journey, her repaired bicycle is finally returned to her, and with her sisters departing on a world tour, she returns to Cerulean City to take over as the active Gym Leader. She remains a recurring presence in later parts of the franchise, appearing in spin-off specials centered on her own story and making return visits during the Advanced Generation, Sun & Moon, and the final series of Ash's adventures, where she once again travels alongside her old friends.

Her key relationships shape much of her development. With Ash, she shares a close friendship built on constant teasing and bickering. She frequently criticizes his recklessness and calls him childish, yet she also recognizes his sincerity and his love for Pokémon, and the two grow to trust each other deeply. The series occasionally hints that she has deeper feelings for him, though these are never confirmed outright. With Brock, her role is largely disciplinary; when he becomes infatuated with attractive women, she pulls him away by the ear and keeps him in line. Her relationship with her three older sisters is complicated; they treat her dismissively and refer to her as the least attractive of the four, which fuels her drive to prove herself and initially makes her reluctant to return home. Among Pokémon, her bond with Togepi is especially central. She becomes the primary caretaker of the baby Pokémon that hatches from an egg found early in the journey, carrying it with her almost constantly and raising it with maternal care. The Pokémon later evolves into Togetic during a visit to the Hoenn region, and Ondine accepts its departure with maturity.

Ondine's development across the series shows a clear arc from hotheaded girl to responsible leader. In the early episodes she is impulsive, quick to anger, and prone to lashing out over minor provocations. Over time, particularly through her responsibility for Togepi and her experiences on the road, she mellows considerably and reveals a gentler, more sensible side. She also confronts a childhood trauma; having once nearly been eaten by a Gyarados as a baby, she carries a long-standing fear of the species. While defending the Cerulean Gym, she overcomes this fear and eventually forms a strong partnership with a Gyarados of her own, which becomes a powerful member of her team. By the end of her main story arc she has grown into a capable and respected Gym Leader, comfortable with her duties and ready to guide younger trainers.

As a Water-type specialist, Ondine has a notable set of abilities and a substantial team. Her better-known Pokémon include Starmie, a fast and powerful battler; Psyduck, an absent-minded partner that becomes a psychic powerhouse when its chronic headache peaks; Goldeen, which she has raised since childhood; Politoed, a cheerful and energetic fighter; Corsola; and Gyarados, which she can Mega Evolve. Her own athleticism is also part of her skill set; she is an accomplished swimmer and has won water-based competitions such as fishing contests and water races. Her greatest asset, however, is her deep knowledge of Water-type Pokémon, which allows her to raise them effectively, devise sound battle plans, and carry on the traditions of the Cerulean Gym.