Gwon Seolhwa, more commonly known by her Japanese name Koyuki Azumaya, is a supporting character in Sgt. Frog. She is a kunoichi, a female ninja, who enters the same school as the main human protagonist Natsumi Hinata as a transfer student. Her past is rooted in a secluded ninja village where she was raised, a background that has left her with exceptional combat and stealth skills but little experience in ordinary modern life. This isolated upbringing also explains her wide-eyed wonder and enthusiasm for the everyday world that others take for granted.

Personality-wise, Koyuki is notably carefree, optimistic, and eccentric. Despite her formidable abilities as a ninja, she has a cheerful and somewhat mischievous streak. Her most defining comedic trait is her inability to keep her ninja identity a secret, as she habitually uses her advanced ninjutsu techniques to accomplish the most mundane of tasks, such as retrieving a dropped eraser or crossing a room. This tendency draws unwanted attention and often causes chaos, but it stems from her genuine, unpretentious nature. She is also intensely loyal to her friends and is driven by a strong sense of justice, believing in protecting those she cares about with all her might. A key personal motivation is her desire to fit in and integrate into standard society, which is why she enrolls in school, even if her efforts are constantly undermined by her own habits.

Her primary role in the story is as a close friend and rival to Natsumi Hinata. Koyuki is initially drawn to Natsumi by what she describes as a familiar scent, and the two quickly become best friends. Their relationship is a cornerstone of her character; Koyuki holds a deep admiration and a clear romantic crush on Natsumi, and Natsumi considers Koyuki to be one of her only true friends. This bond is central to many of the episodes in which she appears. Additionally, Koyuki has a profoundly important relationship with the Keronian soldier Dororo. She found him, then known as Zeroro, after he was injured. Instead of capturing him, she rescued him, taught him the ways of the ninja, and gave him the name Dororo. This partnership is the most stable and cohesive among the human and alien pairs, as they share a positive attitude and a mutual respect for ninja traditions. He trusts Natsumi's family to help guide Koyuki in learning to live a modern life.

In terms of development, Koyuki's arrival marks a significant change from her past. The dissolution of her ninja clan, an event that occurred after she trained Dororo, forced her to leave her secluded life and seek a new existence in the city. Her journey is about learning to navigate that world, from attending school to understanding social norms, all while maintaining her core identity as a ninja.

Her most notable abilities are her exceptional physical prowess and ninjutsu skills. Her athletic ability rivals and often surpasses that of Natsumi, making her one of the most physically powerful human characters in the series. She possesses superhuman agility, stealth, and a highly sharpened sense of smell. She is also proficient in various ninja techniques, or jutsu, which she applies with great creativity, whether for fighting alien invaders or simply for personal convenience.