In the opening scene of the Hero mode, Marie is shocked the Hero does not recognize her as a Squid Sister. It makes sense in multiple ways:
Neither Marie nor Callie have been the Squid Sisters in a while. They've both done their own thing separately. It would make sense no one, but Squid Sister diehard fans, would realize they were in the presence of a Squid Sister.
Agent 4 is not the same age as Agent 3. Even two years of age difference can cause people to have radically different tastes in music, games, movies, etc. Agent 4 may not know of the Squid Sisters or, if they do, may have only heard of them in passing.
The Inklings in Inkopolis Square are more into Off the Hook and never got into the fresh beats of the Squid Sisters, unlike the Squid Sister diehards of Inkopolis Plaza.
A lot of people noticed that Marina's mannerisms and dialogue make her seem a bit uneasy and unsure of herself a lot of the time, like when she nervously adjusts her headphones, or bungles the broadcast while introducing the very first Splatfest. It makes a bit more sense when you consider Marina is an Octoling. In addition to being an immigrant to Inkopolis, she's also living among the people her entire race was just at war with relatively recently. May cross into Fridge Horror when you consider that she may be nervous because she's already been on the receiving end of Fantastic Racism at the hands of less open-minded Inklings, but it remains to be seen how deeply Nintendo will want to pursue that particular plot thread.
In addition, she's a celebrity, even if it's only local to a populous city that would still be a lot of pressure. It seems pretty possible that, while squid kind is very open minded about an Octarian in general, her musical talents would be one of the key factors of that. If she weren't popular, she might not have been accepted as readily. She would not only have the usual pressure to live up to expectations to perform, but also knowing that a big part of positive race relations hang/hung on that popularity.
Likewise is the intro scene where the news of the Great Zapfish and Callie's disappearances are noted. Callie and Marie were both sort of dismissive of it, which makes sense for them, seeing as they are Agents 1 and 2 of the New Squidbeak Splatoon, and are fully aware of the workings of Octo Valley and Inkopolis Plaza's Zapfish disappearance. This time around there's a bit of discord in how to handle the latest activities. Marina is actually okay with heading up a search party for Callie, despite the idea initially being a joke from Pearl. Pearl, meanwhile, insists on leaving it alone like nothing's wrong; given both the above notes on racism and celebrity pressure, Marina acting indifferent to the situation would be out of character for her, to say nothing of the fact that Callie was part of a life-changing experience that led to Marina being in Inkopolis in the first place.
Marina is frequently nervous, anxious, and awkward, which many people chalk up to her being an Octarian in a city of Inklings, or just being her personality, both of which could still be contributing factors. But notice what she does during live shows: when not singing, she's working the turntables and speakers behind Pearl. And during news broadcasts, she's behind a turntable that apparently controls the screen behind her and Pearl, as it switches to a new slide whenever she scratches the disc (and seems to also control the music). She's so anxious and nervous because, unlike Callie, Marie, and Pearl, who've all just had to sing and read the teleprompter, she's doing that on top of tech work that's usually done by completely separate people. She's stressed out from the multitasking, because the chance of something going wrong (and being her fault) is significantly higher.
That's backed up by the story segment the first time you see Off The Hook - Pearl sees their producer freaking out and has to (very unsubtly) remind Marina to check her teleprompter again, which is exactly the sort of oversight she's constantly fretting about.
This actually has more credence as well with Pearl noting Marina as a "geek", which Marina doesn't like being called, that seems to be true. She would be tech savvy enough to run more complicated equipment during a show which would only add on pressure to other factors. After all, she is still only one woman, and while they do most likely have a tech crew helping out she clearly has responsibilities aside from just singing and dancing to memorize during a show.
Marina is a big fan of DJ Octavio. Not only does the sunken scroll reveal that she thought the whole thing was a concert, where she learns about the squid sisters and comes to be fans, but it would make sense if DJ Octavio looked to the Octolings like the Squid Sisters did to the Inklings, a big star who is actually doing something they don't even know about. But look at her preferred instrument, it's a DJ table. In addition she would have most likely based her style on her hero, DJ Octavio, who had a large, flashy rig that he clearly controlled. Suddenly it makes sense why she choose to run parts of the show with technology herself, she saw one of her heroes doing it, and wanted to do it too.
Furthermore, chronologically speaking, her very first song from when she had just met Pearl for the first time (or even from just before that, after having witnessed the Calamari Inkantation performance at the end of the Hero Campaign in the first game) is none other than a solo demo tape of Ebb & Flow, which reuses some Octoling instrumentation as it was the only other music she's ever heard in her life.
Speaking of Ebb & Flow, the evolution of the song itself in the game makes sense:
Marina's original solo demo of it is heavily influenced by Octarian music from DJ Octavio's regime.
The Splatfest version is an electronically beefed-up and 'cleaned up for Inkling audience' version that has had the original instrumentation drowned out by Pearl's vocals and instruments.
The final version, the Fly Octo Fly remix from the final battle in Octo Expansion, is a triumphant and perfected fusion of Octarian and Inkling music sensibilities into a harmonious whole, representing Marina finally becoming comfortable with her life with Pearl in Inkopolis to begin fully embracing her old Octarian cultural roots without fear of speciesism. It also happens to be extremely inspirational for Agent 8, hyping her up enough to help her win the fight against the NILS statue.
Pearl's Japanese name is Hime, from "Hime-Ika" the name of the Northern Pygmy Squid. Pearl's notably short compared to most Inklings because she's a Pygmy Squid. Hime also means "princess," which fits her rich lifestyle and money.
Marina's Black Comedy Burst line about "reality [being] a simulation" arguably came from her time living at home; look at how much of the underground Octarian facilities use panels to simulate some sort of background or sky, or the artificial nature of the modern soldiers including possibly Agent 8. While it's unclear whether she hated it down there, living in such a tech-heavy environment could give anyone second guesses about how real their world is.
In the first game, one of the bosses the player encounters in Hero Mode is the Octomaw. It fights by swimming at the player and coming up underneath them with large chomp, leaving the player to escape the radius of its jaw before they get crushed. In Splatoon 2's Salmon Run mode, there's a Salmonid monster called Maws, which fights by, you guessed it, swimming at the player and coming up underneath them with large chomp, leaving the player to escape the radius of its jaw before they get crushed. It's very likely that, in-universe, the Octomaw was based on the Salmonid.
The salmonids are actually highly appropriate to the changes in this world. Normally, Salmon come from freshwater (which sea life can't live in) but spend their lives in the ocean, preying on sea life. Here, salmonids live in the toxic oceans, and come onto land in maturity to devour what was once sea life.
There are notably less Flooders in both of Splatoon 2's campaigns compared to their prevalence the first game. The Octo Expansion gives a good reason for this—Marina was their main engineer, and she deserted after the first game's final boss!
Conversely, this also explains the presence of various Octarian devices in the Splatfest "Shifty Station" maps, which Marina designs. This is confirmed during the chat logs in Octo Expansion.
The Inklings and Octolings have contrasting personalities and cultures that reflect the Real Life animals they are based upon. Octopuses tend to be solitary creatures, but are amazing problem solvers due to living on ocean floors where there are many different hiding places, prey and predators out and about. Squids, on the other hand, tend to live in groups in the open ocean; they don't exhibit the same problem-solving abilities as octopuses, but they do communicate with each other through flashing different colorful patterns.
Additionally, the more complex, Nintendo Hard station levels of Octo Expansion are a reference to the fact octopuses are much better problem solvers than squids are - using objects (like coconut shells) as tools for later, being able to open jars, navigate mazes, and finding their way in and out of tanks.
Pearl refers to herself with a gratuitous amount of rapping nicknames in Octo Expansion. One of the silliest is "MC Foreign Policy", which is seemingly a baseless non-sequitor of a name. But Pearl, whether she realizes it or not, actually does do a lot of solid work in foreign policy. She helped Marina integrate into inkling society without sparking any cultural backlash. She helped Agent 8 escape her oppressive birth-nation and emigrate to Inkopolis. She even helped eliminate the threat that human technology posed to Inkling-kind.
There is a lot of this oddly-colored ink going around, and it is made by tossing Inklings and Octarians into a massive blender. This gunk is also the ammunition of an absolutely gigantic superweapon designed to kill all the inhabitants of the current world. The creator of this mess goes by the name Commander Tartar. So does that make the goo we have to deal with tartar sauce?
Some of the stage tracks in the Octo Expansion play with Vaporwave techniques and a lot of the Deepsea Metro, Kamobo Co, and Commander Tartar's planet-destroying energy cannon and plan on blending all life into a more perfect life play with its aesthetics. Vaporwave usually samples 80's and early 90's music, the inspiration for Octoling and Inkling music, respectively. Slimiarly, Commander Tartar's actions and intentions represent Vaporwave's Nostalgia but also the dark and unsettling memories that are also from that Nostalgia and never being able to move on from it.
Why does Marina create anime-style portraits of Agent 8 and Pearl, but her representation of the NILS Statue looks sloppy? She likely made the portraits ahead of time, having known what the two looked like, but had to create the picture of the statue in seconds.
In Octo Expansion, the prize you get from defeating the Bonus Boss, Agent 8's memory of Agent 3, is a hairpin that grants the ability Special Charge Up. At first this seems kind of a random reward for beating such a difficult boss, especially considering that Special Charge Up isn't even generally considered that good of an ability. Until you realize it's probably related to how Agent 3 was spamming out Specials at an impossibly high rate for the whole fight.
No wonder why fighting Agent 3 and Inner Agent 3 was very difficult. It makes sense when you played the first game a lot. You are fighting your past self who completed the first game's Hero Mode, possibly reached Level 20+ and/or reached a higher rank in Ranked Battles.
Why is Inner Agent 3's sub of choice the Autobomb? Because they are using the A.I. Breaker exploit that AI Octolings are essentially powerless against them.
The lines about Marina helping to build the Shifty Station levels for Splatfests seems a little random at first, and serves mainly as a Hand Wave for why they're located in Octo Valley. But once you know that she used to be a combat engineer for the Octarian army before defecting it suddenly makes a lot more sense, as she has engineering skills that would be helpful when constructing a stage. Particularly as most of the Shifty Station maps use mechanics and gimmicks from the singleplayer campaign, which is technology that Marina would likely be familiar with.
Commander Tartar's Motive Rant is about how *lings are bastards who fight over small things. But also consider the reputation of online multiplayer gaming culture (especially for team-vs-team games); just look at plaza posts and you'll occasionally stumble across some angry ones ("This weapon is OP!" "Booyah back or else!" "If (Splatfest team) wins I'll throw my Switch in a lake!" "Stop posting Discord links here!"), or look at various official and unofficial forums for games like this one, Overwatch, and League of Legends. And in this game, you play as an Inkling or Octoling, one of countless that Tartar wants dead. Tartar's rant thus sounds like a veiled Take That, Audience! towards gamers.
The addition of Agents 4 and 8 to the Squidbeak Squad comes with a clever little bit of Meaningful Name. Agent 8 is an Octoling... Octolings (and Octopi) have 8 arms. Pretty straight-forward. But what makes Agent 4's title so clever? Well... consider the previous 3 agents (Agents 1, 2, and 3) are all Inklings as well, meaning they're squids. What does 1+2+3+4 equal? And how many arms are squid known for again?
Regarding Commander Tartar and the use of the Colossus of Rhodes:
The Colossus was based on the sun god, Helios. The battle is at a sunset, and the next song is called "Into the Light."
The Colossus was also partially destroyed in an earthquake, and the Greeks didn't fix it because they thought the statue offended Helios. Inklings and Octarians created their civilization and offended Tartar, so it plans to punish them.
The Colossus was only partially destroyed. Parts of it were preserved. Tartar's been alive since humans were, in a way its been preserved too!
Tartar's whole motivation is that it was supposed to find the next evolved, intelligent species, and was disappointed when that turned out to be Inklings and Octarians. But, why didn't it just give them time to improve and evolve further? It was made by humans, and human ego says we've evolved fully. For Tartar, the peak of evolution is the point humans were at when it was made.
The names of Off the Hook's songs say something about the artists:
Color Pulse: All Octolings use magenta. When Marina defected, she chose to switch to cyan. Color's important to her.
Ebb and Flow: The title refers to the back-and-forth movement of liquid. Marina is actively trying to imitate Marie, and what was her solo song called? Tide Goes Out.
Muck Warfare: While Inklings shoot ink for fun, Octarians shoot to kill.
Nasty Majesty and Acid Hues carry the more serious meaning of shooting ink than Inklings have.
Fly Octo Fly: This song is sung as Pearl's vocal warm-up, and the name sounds like something an immature rapper like her would use.
Why does Octavio use a remix of Bomb Rush Blush instead of I Am Octavio, or a new song? Is he lazy? Part of some slander propaganda? Here's an interpretation: He's lonely.
The Knights vs Wizards Splatfest sees Marina on Team Wizards. Makes sense — in a lot of Final Fantasy-esque video games, the strength of magic-attacks is measured by a character's Intelligence stat, which happens to be Marina's strong suit. By contrast, Pearl's vocal chords' sheer volume is probably the closest Splatoon-equivalent to a character with a high strength stat.
Despite being a popular idea, there's actually a good reason the final Splatfest isn't Pearl vs. Marina: The Splatfest usually comes down to two different ideals between the two hosts, and Pearl openly acknowledges that Marina is the more popular of the two Off The Hook members, effectively crowning Marina the winner of said Splatfest if it existed.
Why is DJ Octavio on the opposite side from his Octarian during the Final Splatfest? It makes sense giving their motives. Octavio is bitter, resentful and wants revenge on the Inklings and doesn't care who he harms or how much Chaos he inflicts, while the Octarians have bought into the propaganda perpetuated by Octavio and really just want the Order taking some of the land owned by the Inklings might possibly provide compared to their crumbling home.
In the Salmonid Field Guide, it states that the Gushers that Salmonid war parties can erupt from (as well as their Goldies) are technology acquired from a trade deal with the Octarians. This ultimately begs a single question: what do the Octarians get out of this? Aside from whatever may be related to the Octomaw, what is in it for the Octarians to fund a species perceived to be Always Chaotic Evil? Are they just venting their excess inventory to save themselves, regardless of the buyer? Or is Octavio trying to force the Inklings into a two-front war, using the Salmon Runs as a distraction to attack Inkopolis?
There's another option: remember that "standard" power eggs have been in Octarian domes since S1. At the time, it just seemed a bit odd and you didn't think much of it... but those eggs aren't Octarian. They're Salmonid. Absent access to the best parts of the sea and major Zapfish, the Octarians are forced to try trading with the Salmonids for any kind of power source to keep the domes going. So not only are the Octarians having to risk contact with the Salmonids in general (as it seems likely that Salmonids find Octos just as tasty as Inklings) and are only getting a deal with the Golden Ones out of convenience that helps the Salmonids hunt Inklings better, but given the place of Salmonids in Inkling history and culture, the Octarians have no choice but to commit an act that the Inklings will never forgive if knowledge of it becomes widespread. The Octarians don't do it because they want to - they do it because they have to.
Indeed, the developers have confirmed that the Octarian trade with the Salmonids primarily to acquire their power eggs.
And about all this: once you beat Octo Expansion, you can take your octo-kid into Salmon Run just like an Inkling, and Pearl notes in Chat Log 10 that a number of Octolings are joining up for it. So... young Octolings are now working against the Salmonids. Are the kids inadvertently torpedoing the deal their elders set up with the Golden Ones? What kind of consequences is that gonna have?
Pearl and Marina sometimes reference their own shifts as Grizzco Salmon Run employees. At first this just seems like a background gag, encouraging players to go join in themselves. But if the Octo expansion reveals that Marina is a former weapon designer for the Octarians... the very same group now arming the Salmonids. She's not doing it for the money, she's doing it to keep tabs on something that could potentially annihilate her new home and might very well be partly her fault. And Pearl tags along because she's her friend.
Fun fact: In real life, salmon eat squids. This puts their use of cookware-based weapons in a much darker light...
It also puts Grizzco in a darker light. Ever wonder why the Salmonids come up onto land and fight you, allowing you to kill them and take their eggs, when they'd be perfectly safe if they stayed underwater? They come up because they're hungry for your flesh. Grizzco is basically using the inklings as bait. Just like regular fishing bait, if the hooks were ink guns, and the bait actively wielded them against the fish..
Recall that in the lore the Octarians are stated to donate tentacles (that regrow) in order to grow more of their kind. Now, think about why Agent 8's top is asymmetrical, missing onesleeve.
The enemies Agent 8 fights against are Sanitized Octarians - they're "sanitized", likely against their will, which removes them of any free will and brainwashes them. This sanitizing is done in a test facility, meaning these Octarians are experimented on.
When Agent 8 is splatted during the escape sequence of the Octo Expansion, their mission control acts much more dramatically, outright screaming their name. This doesn't occur at any other point of the expansion, much less the rest of the game. The reason can be found when they respawn. That is to say, they don't. Instead of the little ghost going back to the respawn station, the camera cuts to Agent 8 standing on the checkpoint like nothing happened. And whereas in the rest of the expansion, your mission control crew remembers you getting splatted before repeated attempts and will generally have new dialogue to match, they give no indication of this during this section and fully repeat anything they saidnote the sole exception being the laser sequence, where Marina's dialogue will change to outright tell you how to avoid some of them, and even then, this isn't portrayed as her remembering your death. At any other point in the game, respawning has the in-universe justification of the player actually being linked up to spawn points. At this point in the story, Agent 8 is going off the rails Portal-style and is no longer connected to these stations, so if they die, they actually die, with "respawning" now being replaced by your standard out-of-universe time rewind. An interview with the game's producer later confirmed this to be the exact case.
This is even worse during the NILS statue stage, as there is no respawn platform on the map. So if Agent 8 falls into the water and dies, not only does their mission control freak out but the losing cutscreen plays out anyway as if they ran out of time before detonating all 30 Hyperbombs.
Octarians had dedf1sh, an up-and-coming star DJ Octoling who sought to mix the best beats ever heard. Well, Commander Tartar got ahold of her and, like so many other abducted Octarians, sanitized her. Just enough of herself remains that she still mixes music, and it's described almost like being an autonomous zombie-like impulse to her, obsessively doing the only thing the last scraps of her recall.
Here's the real fun part, the reason her existence was revealed? It's because the music you hear in the Deep Metro test chambers is her playing from somewhere far away. However, you never encounter her. Agent 8 and 3 make mincemeat of Tartar's machinations and escape with Cuttlefish back to the surface, leaving those test chambers to never be used again. In other words, dedf1sh is still down there somewhere, alone in the darkness, perpetually playing her music for nobody in a desperate attempt to cling to the last vestiges of her original personality and memories.
Remember how Tartar said that the teal sludge was made from "the best and brightest test subjects?" The same teal sludge that's used as ink by its army? Ink that makes physical contact with Agent 8 multiple times? When you think about it, Agent 8 has been getting covered in what amounts to a mixture of liquefied skin, organs and countless other tissues for the entirety of the Octo Expansion.
And as if that weren't horrifying enough, Agent 3 uses the sludge as ink during their brainwashed boss fight. Knowing what we do about how Inkling ink works, that means the stuff had to have been flowing through their body. 8 was merely bathed in corpses; 3 was basically injected with them. [[Squick Yuck...]]
The first Splatfest after the Octo Expansion released is whether or not orange juice should have pulp in it. Given that Agent 8 just narrowly escaped being blended alive, turning a solid into a liquid will be a touchy subject at best and a PTSD trigger at worst.
Remember how a few times Pearl and Marina state that the Splatfest results are actually put into law. The second Splatfest after the Octolings move into Inkopolis is... Squid vs. Octopus, with the Inklings and Octolings dividing along species lines. Given the fact that these two factions were once at war, and the Octolings are only just beginning to re-integrate into Inkopolis, having the two groups fight to have the winner legally declared superior is... a little iffy. Granted, Pearl and Marina were likely just joking about the laws changing to fit the Splatfest, but even so...
While the final battle and the pre-fight cutscene of Octo Expansion occurs, keen readers will notice that both the stars of Off The Hook will both mention the "end of the world." This could be just some coincidence at first, seeing as it's a common term for the apocalypse, but it's not. It's been made clear that only a small portion of land on Earth is even habitable at this point, meaning that the landmass Inkopolis is part of could be the only thing they have left for the entire population of Earth. Just wiped out by Tartar as if it were nothing.
The Octobosses return for the Octo Expansion, except sanitized. And they're confirmed to be the same ones that were originally fought in the original single-player, based on titles like "Revenge of the Octo Samurai" and the Neo Octostomp's dialogue of "Back Like I Never Left". Sure, they're much more of a pain in the ass in the expansion than in the original single-player, but based on what is known about "sanitization", did they really deserve that to happen to them?
In the Frosty Fest, Pearl is team Spend the Holidays with Family while Marina is Spend the Holidays with Friends. Which makes sense, because Pearl talks a lot about her wealthy family, while Marina... she can't spend holidays with her family. They're still, at the very least, stuck in the despotic, desperate Octarian state from which Marina is a refugee, if they haven't outright been punished for their daughter's defection.
In the lore it's mentioned that Octolings occasionally donate tentacles that grow into other Octolings. The details are vague so we don't know the full extent of how that works, but it also leaves open the idea that Marina doesn't have a family to spend the holidays with even in Octo Valley, and Pearl is genuinely the only "family" she has.
In the Super Smash Bros. Ultimate Splatfest, Marina is on Team Villains. When Pearl asks about it, Marina says that that's because villains "aren't bad guys. They're just...misunderstood", and that "Villains want to fight the establishment and topple the status quo. But they're branded as "villains" just because they want to change the world." Marina is an Octoling, a species that most of Inkling society considers to be evil, and even within her own species there's probably quite a few who consider her a traitor for defecting to live amongst Inklings. No wonder she would pick Villains.
Commander Tartar's directive is to coat the entire Earth with its very own mush, then how would it destroy the entire population of Inkopolis? It's been shown that even to Agent 8 note albeit not when the Escape Sequence or Final Boss occurs the sanitized ink still can respawn those who have been splatted, so what is stopping from one just respawning? Especially considering all spawn points in Inkopolis and Octo Valley/Canyon have fields that block ink. So unless Tartar was planning on constantly barraging Inkopolis for years on end, this plan is half-baked at best. At the most, it'd just turn Inkopolis into a wasteland but not an absolutely uninhabitable one.
Spawn Points probably have a maximum amount of Inklings/Octolings that can fit on it, with anyone else getting shoved out of the ink-blocking field by sheer volume, where they'd land in the sanitized ink. I doubt that anyone would have time to acclimatize to the ink either, given that. Or it's just Gameplay and Story Segregation, either one.
How is Tartar's NILS going to take care of the still-aquatic creatures like Jellyfish and Salmonids? No ink, not even the one used in Octo Expansion can exist in water.