Agent 4

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

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 the new version of the Hero Suit, 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.

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.

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.

Trivia

 * Based on Marie's dialogue stating that Agent 4 is "two years older" than Agent 3, one can assume that Agent 4 is at least 18 years old, as Agent 3 was about 14 years old around the events of, and would be around 16 years old during the events of.
 * This has been a topic of confusion and debate within the community, with many believing Agent 4 to still be 14, noting the inklings' similar physical appearance to those of the first game. Marie's statements which would make Agent 4 roughly 18 years old also contradicts a statement made in an interview with Splatoon 2 producer Hisashi Nogami that "The previous playable Inklings would be around 16 now, so the new 14 year old Inklings will step up to the plate. The playable Inklings will still be around 14." It is unknown if this is an oversight or not.
 * It has also been suggested that Marie's dialogue is a meta joke, directed not towards Agent 4, but towards the player who controls Agent 4 themselves.
 * It can be presumed that Agent 4's canonical gender and appearance is that of a female inkling with short tentacles, as there are no available images of a male Agent 4, and all promotional art has depicted Agent 4 as such. This is in contrast to the first game, which had multiple official art depicting both a female and male inkling as Agent 3.
 * This is further evidenced by the final portion of the end credits, which depict 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.
 * Agent 4's ink color varies based on the mission, it is also a specific color depending on each sector of Octo Canyon. They have yellow ink on Sector 1, green ink on both Sector 2 and Sector 5, and teal ink on both Sector 3 and 4.