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.
Appearance
Splatoon
Due to them being a customizable character in Splatoon, their appearance and gender can be changed. During Hero Mode they wear a Hero Suit and armor along the way that can be broken. Their gear consist of the Hero Shot, Hero Charger or or Hero Roller with the last two being used in amiibo challenges. This appearance also shows up as an alt costume with yellow hair in Super Smash Bros. Ultimate with a squid sticker on their ink tank.
Fig 1. Inkling Boy Agent 3 with the Squid Sisters, Fig 2. Inkling Girl Agent 3 with the Squid Sisters
Octo Expansion
Like in Splatoon, Agent 3's appearance can bee changed but with the same hairstyles from the original Splatoon and now wears a cape. Under mind-control their hair and glowing eyes is a turquoise green with a gelatin like substance around the right eye of the same color.
Mind-controlled Agent 3s.
Splatoon 3
Now with the title of "Captain" the appearance is chosen for the player if they played Octo Expansion (where their appearance was first chosen by player on a Switch game) and let Splatoon 3 (also on Switch, this can be changed too anytime. If no save data is carried over, they have the feminine style with the third-lightest skin tone and green eyes.
They now wear Cap'n Cuttlefish's cap with multicolored tattered clothes over a grey shirt. The scruffy look can be seen with their hair too with scars on both tips. Their black pants are also tattered and has flip-flops that look like their shoes from the Hero Suit.
Their sitting pose in Splatoon 3.
External Links
Agent 3 (Splatoon) - Inkipedia, the Splatoon wiki