Koala is a former slave from Foolshout Island who was liberated by the fish-man Fisher Tiger during his raid on Mary Geoise to free the captives of the World Nobles. While traveling with the Sun Pirates to return to her home island, she overcame her initial fear of fish-men and developed a deep bond with them, serving as a bridge between humans and fish-men. Her experiences aboard the Sun Pirates left a lasting impression on her companions, particularly Jinbe, who saw in her lack of prejudice a reason to reconsider his own disdain for humans. After returning home, Koala eventually left to join the Revolutionary Army, an organization dedicated to dismantling the World Government she so despised.

Within the Revolutionary Army, Koala rose to the rank of a high-ranking officer. She trained extensively in Fish-Man Karate, mastering a martial art typically exclusive to fish-men, and became an assistant instructor for the skill within the army. Koala is a close friend of Nico Robin, and she works alongside the chief of staff, Sabo, during critical operations such as the events on Dressrosa.

Motivated by the injustice she suffered and witnessed as a child, Koala is a passionate, empathetic, and determined fighter. She is fiercely loyal to her friends and to the cause of freedom. Her development is marked by her transformation from a small, terrified girl sold into slavery into a confident and powerful woman who actively fights against the very system that once imprisoned her. She embodies the reconciliation between humans and fish-men, and her abilities in Fish-Man Karate make her a formidable combatant capable of manipulating water to fight on equal footing with far larger opponents, a skill she often puts to use in the Revolutionary Army's campaigns against tyranny.