Inner Agent 3

Inner Agent 3 is an extra boss in Octo Expansion.

Inner Agent 3 is unlocked by collecting all eighty full mem cakes by beating each level legitimately without asking for help and completing the escape levels without skipping the final level. The boss fight is initiated by investigating locker 8 near the vending machine in Central Station. The player is rewarded with the Golden Toothpick after defeating the boss.

Gameplay
The boss behaves like the Agent 3 battle in the Spinal Phase, but with significantly increased health, accuracy, and aggressiveness; the arena itself is also much smaller and the walls and cover present cannot be inked.

In all phases of the fight, Inner Agent 3 is equipped with as their main weapon and s as their sub weapon. Inner Agent 3 can perform a dodge roll at any time as if they were landing with the Drop Roller ability. There are five phases, and the player must disable their shield to progress past a phase. In each phase, Inner Agent 3 uses a different special weapon. Inner Agent 3's shield can be disabled by shooting them ten times, or by detonating a Splat Bomb near them and then shooting them four times, or by detonating two Splat Bombs near them. Inner Agent 3 will recover from damage over time if the player does not attack them, accelerated if they swim through their own ink.

Phase 1
Agent 3 will enter the battle with a. During this phase, Agent 3 will attack with the to supplement the Hero Shot and Autobombs. While using the Inkjet, Agent 3 will be able to fire a maximum of six shots before returning to the launching point, which is marked just as it is in multiplayer. While they are vulnerable during the special, it is equally important not to stay put long enough to take damage in the process.

Phase 2
Agent 3 will Super Jump into the stage with a Splashdown. During this phase, Agent 3 will now use the, which will both protect them and let them cover large amounts of turf at once. However, Agent 3 is exposed and cannot fire back when using it, though they may still perform a dodge roll. Be cautious as Agent 3 will oftentimes follow the Bubble Blower with an Autobomb which automatically explodes upon hitting a bubble, causing all nearby bubbles to burst.

Phase 3
Agent 3 will once again start off with a Splashdown Super Jump. During this phase, Agent 3 will use an, deploying up to four Autobombs at a time. As the player will likely be surrounded by Autobombs if Agent 3 is left alone, taking them down quickly is advised. Unlike with the previous specials, Agent 3 can still use their Hero Shot while Autobomb Launcher is activated.

Phase 4
Agent 3 will remain on the floating platform during this phase, using the. Between specials, Agent 3 will fire onto the field or throw Autobombs. While the Octo Shot's fire can sometimes reach Agent 3 if the player fires while jumping, the only way to deal with this phase reliably is to throw Splat Bombs in fairly close succession, thus dealing more damage than Agent 3 can heal from.

Phase 5
Agent 3 uses a similar sequence of three Splashdowns as they did in the earlier boss fight, but the Splashdowns are executed much faster with no openings between each use. Once enough time has passed, Agent 3 can use their chained Splashdowns again. Notably, once Agent 3 has maintained enough damage, they will automatically recover their special and may use a single panic Splashdown. In order to avoid getting splatted or taking damage from the Splashdowns, standing on the opposite side of one of the two obstacles on the map will help to block any damage. Beware of losing turf control at this point, as being slowed by Agent 3's ink will likely be fatal.

Trivia

 * It is unclear whether the battle actually happened, or was simply Agent 8's thoughts as the result of collecting all mem cakes and therefore all their past memories. It is unlikely that Agent 3 would actually battle Agent 8 since they have no given reason, supporting the latter theory.
 * Agent 8 was, according to Cap'n Cuttlefish, in a battle with Agent 3 before the events of Octo Expansion, but never got to finish after they got taken by "someone". This might mean that Agent 8 is mentally continuing the battle as a result of getting their memories back, although whether Agent 8 did this willingly or not remains unclear.