Agent 4

Agent 4 is the codename given to the player character of by Marie in Octo Canyon.

Appearance
Agent 4 can have a variety of appearances, depending on the player's choice of gender, skin color, eye color, and hairstyle, as well as their current ink color. They wear an upgraded version of the old Hero Suit that Callie used to wear and can pick up Armor to wear as well. They wield the Hero Shot by default but are later able to use a variety of other Hero weapons given to them by Sheldon.

No matter what the player is wearing, when Agent 4 enters Octo Canyon for the first time, they wear the White Headband, Basic Tee, and Cream Basics before shortly getting changed into the Hero Suit by Marie.

Personality and traits
Agent 4 is described as a silent squid who goes with the flow even in raging waters, and values freedom and flexibility. They also "blast through any situation with ease." Agent 4 did not initially know who the Squid Sisters were, but later came to respect them. Agent 4, as seen in promo pictures and art, has a diligent work ethic and a lot of confidence. Unlike the Captain, they are more cheerful and upbeat in their mannerisms, and almost always smile.

History
Agent 4 initially went to Inkopolis for an unknown reason but got swept up in Marie's quest to save the Great Zapfish and her cousin Callie. In Octo Canyon, Agent 4 completes various missions to retrieve stolen Zapfish, defeating many Octarians along the way. Eventually, Agent 4 confronts DJ Octavio and a brainwashed Callie, rescuing the Great Zapfish, freeing Callie, and finally restoring Inkopolis' power supply.

Trivia

 * In the Splatoon manga, Gloves is Agent 4, with Skull and Vintage appearing later as Agent 4.1 and Agent 4.2, respectively.
 * Marie dialogue exclusive to certain localizations suggests Agent 4 is "two years older" than Agent 3, suggesting that Agent 4 is 16 to 18 years old. This directly contradicts the developers' statement that the playable Inklings in Splatoon 2 are 14 years old.
 * Most promotional artwork and screenshots depict Agent 4's gender and appearance to be that of a female Inkling with the Haircut hairstyle, although some concept art and a segment of the credits depicts a male version of Agent 4 with the Slick hairstyle. This is less variable than the first game, which had multiple official artwork pieces depicting both female and male Inklings as Agent 3.
 * This is further evidenced by the final portion of the end credits, which depicts a female Agent 4 celebrating alongside a reunited Callie and Marie.
 * One of Marie's dialogues when spoken to in Octo Canyon reveals that she finds Agent 4 to be "pretty adorable".
 * She also states that Agent 4 has much better hygiene than Agent 3, but only in the English version.
 * Agent 4's ink color varies based on the mission, while it is also a specific color depending on each sector of Octo Canyon. They have orange-yellow ink in Sector 1, green ink in Sectors 2 and 5, seafoam green ink in Sector 3, and Sector 4 is teal ink.
 * A female Inkling on the artwork for Tower Records has the same features commonly used for Agent 4. She is seen with an Octoling that resembles Agent 8, implying it could be both of them.
 * Agent 4 is nicknamed "Four" and "Agent Snore" by Marie.
 * According to the Official Splatoon 2 relationship chart and The Art of Splatoon 2, Agent 4 does not watch TV because they're busy with extracurriculars. Therefore, they are unfamiliar with the Squid Sisters.
 * This explains why they stood in awkward silence when Marie brought up the subject of her fame.
 * The Octo Expansion art book, HaikaraWalker, includes a mid-development mockup of Agent 4 in a prototype version of the Deepsea Metro.
 * A map of Inkadia reveals that Agent 4's home is very far away from Inkopolis.
 * In The Art of Splatoon 3, Agent 4 was shown to be in the earlier concept of Splatoon 3's story mode

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