How Sanitization Works
(by a half-sanitized Octarian)

Sanitization... well, the way sanitization impacts you depends entirely on the type of the sanitized ink. There are three different types of ink - non-diluted, 50% diluted, and 75% diluted. Dilution is proportional to how much water is mixed into the ink. (This is the way inkfish ink works, too - ink used for regular battles is generally 75% diluted.) All three forms of ink are applied via submersion, but non-diluted sanitized ink must be injected as well to work.

The sanitized enemy Octarians in Kamabo Co. are the ones who use the 75% diluted sanitized ink. It funtions pretty much the same as regular ink, but it is dangerous and will hurt inkfish if you get too much of it on you. It's kinda acidic and eats through membrane really easily. It's painful in a way that normal ink isn't, but it's not gonna turn you into a sanitized person, it's not strong enough for that.

50% diluted sanitized ink is what Commander Tartar used to extract the memories of test subjects. There's a massive basin of this ink above the portion of the Deepsea Metro that Kamabo Co. operates in - that's where the character creation screen in Octo Expansion is. It doesn't take away your soul - which, in Splatoon, is both the thing that makes you "you" and the thing that allows you to respawn, which is literally coming back from the dead, by the way. 50% diluted sanitized ink takes away your memories (which are then made into memcakes and stored in a Kamabo Co. facility), but it doesn't take away your soul, and it doesn't impact your physical appearance more than making your irises green. You're still alive.

Non-diluted sanitized ink is used on failed test subjects to "fully" sanitize them. A fully sanitized Octarian retains neither their memories nor their soul (meaning that they can no longer respawn when splatted) and are considered clinically dead. Fully sanitized Octarians don't need to eat more than maybe once a month (although they still can eat more if they want to), don't breathe, don't have a pulse, can only produce 75% sanitized ink rather than normal ink, have green skin and blue-purple tentacles, and usually smell faintly of bleach. The smell comes from the special upkeep that sanitized Octarians need to do on their body to keep it from rotting alive. C. Tartar wasn't really doing that upkeep in Kamabo Co. - or he was only doing the bare minimum - so it's common to come across sanitized Octarians who are missing a limb or other parts of their body.

A freshly sanitized Octarian has lost all sense of self and is completely maleable to the whims of anyone around them. They aren't physically brainwashed, it's not some sci-fi mind control thing, it's just that sanitized Octarians don't know anything, and so if someone tells them to do something, they'll kinda just do it, no questions asked, because what else are they going to do? They know no reason to disobey. Also, while they don't have any actual memories, things like muscle memory still remain, which means the sanitized Octarians are still very useful in Kamabo Co.'s army.

A sanitized Octarian that has escaped the Metro and returned to the surface can slowly regain their sense of self (or at least build a new one) and become a functioning Inkfish again. There is currently no known cure to sanitization, but after the events of Side Order, Marina did continue working on the Memverse as a way to "cure" sanitized Octarians. However, this cure will only fix the mental aspects of sanitization, not the physical.

Agent 3 was a special case, because they were sprayed with non-diluted sanitized ink. However, since they hadn't been submerged in it nor injected with it, the effect it had on them was small and temporary. Only the portion of their face were the ink was was turned green and "dead", while the rest of their body was unimpacted by the ink. The mental effects of the ink were also limited - their memories were temporarily innacessible, but not removed, and their soul was also mostly unaffected. They were still easily persuaded to do things they normally would not, though, which is why they obeyed the commands of C. Tartar. However, their natural limiter was also temporarily "deactivated" by the ink, which allowed them to push their body far beyond its limits in battle, which permanently disabled them after the fact. Going with our half-sanitized/fully-sanitized terminology here, Agent 3 would be considered 1/3 sanitized.

Most Octarians who came out of Kamabo Co. after Octo Expansion were fully sanitized and had to go through rehabilitation programs funded by Off the Hook to regain their sense of self and unlearn the harmful ideology that C. Tartar had poisoned their minds with. A few of the Octarians who came out were only half-sanitized, having been in the middle of the Kamabo Co. program when C. Tartar was defeated, or having been one of the few Octarians who "gave up" and stayed on the Metro forever, too scared to risk their lives in another challenge. However, these Octarians are few and far-between, and are much more rare than fully sanitized Octarians. Agent 3 is the only sanitized Inkling alive.