Agent 4

Agent 4 is the role the player is tasked in Octo Canyon, assigned by Marie.

Etymology
Similar to how Agent 3 is called so by Captain Cuttlefish because Agent 1 and Agent 2 are Callie and Marie, Agent 4's name is given by Marie in continuation to Agent 1 (Callie), Agent 2 (Marie), and Agent 3 (the player in the original Splatoon). Once again, the player's real name is never revealed.