Overview
Agent 3 (3号, San-Gō, lit. Number Three) or Captain is the protagonist of Splatoon's Hero Mode, they have no fixed appearance as their appearance reflects the player's character, but she is often depicted as the long-tentacle haired squid girl from key artworks.
When they first visits Octo Valley, Cap'n Cuttlefish forces them to join the New Squidbeak Splatoon.
In Splatoon 3 they are given the tile Captain of the New Squidbeak Splatoon when Cap'n Cuttlefish retired.
Note that while this squid is now a captain there are others after them holding the Agent 3 title such as New Agent 3.