Agent 3 is an inkling, but their appearance is subject to the player's choice. Most commonly, they are depicted as they appear in promotional material, in a character officially dubbed as VeronIKA. That is, a girl with long, asymmetricalblonde hair, with a single braid, and yellow eyes.
They are most often depicted with the tri-stringer or hero shot weapon, and an ink tank on their back. They are also often accompanied by a Smallfry companion.
Their outfit changes during gameplay; some examples can be seen in the images below.
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