The story follows a young woman named Kasumi, who is a cyborg ninja of the year 25XX who works for a secret organization known as NEW (New Eternal World), a terrorist group which plans on conquering the world. As Kasumi continues her work in the organization as NEW's top assassin, she fights against the governments of the world for NEW to reach their goal as the ultimate rulers.
Kasumi - The protagonist of the game, a kunoichi who was rebuilt as a cyborg by the NEW organization after she was almost killed by her master.
Nick Freeman - The charismatic leader of the NEW organization and the person who saved Kasumi's life from her master.
Dr. Draco - The doctor and mechanical engineer who saved Kasumi's life and treats her like his own daughter, he was forced into working for NEW.
Saizo - A mysterious ninja who wears a black fox mask and always fights against Kasumi whenever they meet.
President Guy Welkinson - The president of the United States of America in year 25XX, a strong-willed and charismatic peacemaker.
Kasumi develops its narrative thread through the actions of its player-controlled protagonist, Kasumi. Viewed from a third person over-the-shoulder perspective, in typical action-adventure fashion Kasumi starts the game with basic, low-level abilities and weapons that can be upgraded as she progresses, by discovering or buying items. In keeping with her ninja persona, her character can interact with the game environment to perform acrobatic feats, such as running along and jumping off walls, swinging from pole to pole, or running across water.
Inspired by the mechanics of the The Legend of Zelda video games, the access to the next stages are obtained by fighting enemies, finding keys, or solving puzzles. Through out the game, the progress can be saved anywhere at anytime where a battle isn't occuring, but there are also glowing butterflies known as Spirit Butterflies floating around near upcoming boss battles, these Spirit Butterflies can be used as a healing point and saving spot. In game cut scenes will appear from time to time during gameplay, some cut scenes are optional and can be skipped without anything happening to the original story; the Spirit Butterflies are a reference to the Tsukiakari game series.
In both Story and Mission modes, game scoring is based on the player's speed in clearing encounters, the number of kills achieved, the number of unused ninjutsu remaining at the end, and the amount of cash collected. The points at the end of a stage is turned into money which can be used in the shop to buy hidden ninjutsu skills, weapon upgrades, throwing and healing items, or costumes. Costumes for Kasumi can be unlocked by achieving optional cut scenes in Story Mode or completing certain missions in Mission Mode. Some unlockable costumes are based off the clothes of other Aozora characters.