Protagonist of Splatoon's Hero Mode.
They are recruited into the New Squidbeak Splatoon by Cap'n Cuttlefish to help retrieve the zapfish from the underground civilization of Octo Valley.
Due to Agent 3 being the player character, they can be depicted as either gender, though they are often drawn as a girl with green tentacle hair thanks to promotional materials surrounding Hero Mode. Regardless, Agent 3 always wears a "hero suit" consisting of a bright yellow safety vest and pointed headgear.
By the time of Splatoon 3, they have succeeded Cuttlefish as the captain of the New Squidbeak Splatoon. In this game, they are now simply known as "Captain", while a new agent inherits their former "Agent 3" callsign. As the Captain, they wear the same blue peaked cap that Cuttlefish used to wear.