A reader had several pointers here about how Monster X's silver theme is so appropriate in the story beyond the Silver Bullet trope.
AbraxasVerse Ghidorah's plan for Vivienne explains a lot of the actions Ghidorah took throughout the Antarctica scene in King of the Monsters, as pointed out by a reader. Such as why Ghidorah doesn't change targets from the downed Osprey to the airborne Osprey even after Madison's use of the ORCA on the flying Osprey should have hit Ghidorah's Berserk Button, and it also fits how Ichi/Eldest Brother doesn't use his Gravity Beam again until the Isla de Mara scene (by which time Vivienne is presumably no longer in Ichi's neck and has been safely transferred to the "womb" in San's neck, which also lines up with San not using his Gravity Beam at all throughout that scene in the movie).
Ghidorah slowly crushing the downed Osprey in Antarctica is the only time in the movie that Ghidorah displayed this brand of cruelty – the main ways its sadism manifested throughout the rest of the movie were its Kill All Humans attitude and overkill usage of its Gravity Beams. The reason in Abraxas' context why Ghidorah opted to slowly crush the Osprey? It was probably trying to flush Vivienne out of there. The author herself admits she didn't think of this at the time, but she seems to haveaccepted and canonized it.
When the G-Team shot at Ghidorah and the heads all paused for a time, with San curiously nudging them whilst Ichi/Eldest Brother was visibly scanning them; San's behaviour is entirely in-character, but why were the other two heads pausing, considering that Abraxas establishes they sorely lack San's curiosity? Because Ichi/Eldest Brother wanted to make sure Vivienne, the one human he wanted to grant a Fate Worse than Death to (for which he needed her body intact) wasn't among the G-Team's numbers before atomizing them.
San and Vivienne have no track of time during their imprisonment, but it appears their reconciliation doesn't come long before their escape (which Chapter 7 reveals occurs roughly in the middle of May). Considering Vivienne's birthday is April 27, just before May; it's possible her and San's reconciliation while imprisoned happened on Vivienne's birthday.
In Chapter 4 of the main fic, San has a very bad gut feeling when he sees Mariko and Jonah's men wearing full hazmat suits. After reading "Damnatio Memoriae", it seems feasible to guess that the sight of the humans in those suits stirred his faded memory of the similarly-suited Makers that originally captured and tortured San and his brothers.
Ilene's Past Experience Nightmare of Vivienne's death scream in Antarctica is apparently the first time she's had that dream in a while. Considering the timing and considering that Mothra was able to sense Vivienne's Howl of Sorrow from her shrine in China, it's entirely possible that said Howl of Sorrow coupled with the Chen twins' implicit psychic abilities triggered the dream.
Jonah thought he could use Ghidorah or Monster X's Healing Factor to bring back one of his dead loved ones, either Asher or his daughter. We know it's most likely the latter because Ash's body went down with Outpost 32 when it collapsed into a chasm during Ghidorah's awakening, so unless Jonah believed something with Ash's DNA on it like a belonging would be enough to restore him with Ghidorah's abilities, there was simply no physical trace of Ash's DNA to work with – Jonah's daughter however might have left a body in a grave, or an old belonging with DNA traces on it such as a Tragic Keepsake which Jonah had access to.
The observation on the Fridge page forGodzilla: King of the Monsters, that the Legendary version of Ghidorah has sideways-facing eyes which imply Ghidorah is a prey animal in its natural environment, ties in nicely with Ghidorah's backstory in Abraxas. It's clearly confirmed that the Dorat-adjacent alien wildlife species which AbraxasVerse Ghidorah originally came from were preyed on by other animals in their environment when they were young, so Ghidorah's sideways-facing eyes are probably a remnant of that.
Ghidorah's backstory of having begun as an adaptation of the Dorats could explain a bit of Fridge Logic in King of the Monsters: why does Ni/Elder Brother have a perpetual chipped horn, which Ghidorah's Healing Factor doesn't fix up even when said Healing Factor is activated multiple times in the movie? The likely answer is, Ghidorah's Healing Factor was programmed into its biology by the Makers rather than an ability that the trio's original forms possessed, and Ni's horn got chipped before he became part of Ghidorah. This is pretty much confirmed in the Abraxas: Empty Fullness one-shot "Damnatio Memoriae".
Manda's egg conveniently hatching almost as soon as it's exposed to sunlight has happened before in a manner, in Godzilla (2014) with how Hokmuto's egg was catalyzed by exposure to the outside world's air. Manda's embryo inside the egg fully developed a long time ago, and it was waiting in dormancy for a catalyst to trigger his hatching just like the buried, millennia-old MUTO eggs were.
San and the dragon whose voice he has (Grigori from Dragon's Dogma) are both genocidal dragons who only act so destructive because they're essentially Forced into Evil, not because they choose of their own volition to be so.
The late Emma Russell remaining unforgiven by Madison for her atrocities becomes posthumous Laser-Guided Karma if you consider one particular detail from the King of the Monsters novelization canon to AbraxasVerse: in the novel, Emma was convinced that she'd be Easily Forgiven by Madison of all her crimes if she cleaned up the mess she'd made with Ghidorah.
San greeting the photographers by Flipping the Bird gains a layer of Fridge Hilarious. Due to San's origin and due to Ghidorah never bothering to directly communicate with humans during its rampage, this event is essentially the public's First Contact with a non-hostile extraterrestrial intelligence – and the alien's way of saying he comes in peace is by flipping us off!
Being swallowed whole by a Kaiju Big Bad isn't the only thing Vivienne has in common with Bill Randa from Kong: Skull Island in this fic – both of them had Arbitrary Skepticism to the idea of aliens, and both were ultimately disproven on that front.
San means Three in Japanese. San's Evil Doppelgänger has the Fan Nickname San-2. Three-Two. Get it? Like Outpost 32.
Titanus Scylla's first appearance foreshadows the reveal of Ghidorah's Paradise Lost-inspired plan for Vivienne in the very next chapter: Scylla's namesake in Greek mythology looks like a woman with multiple dogs emerging from her belly, which also happens to be how Sin is portrayed in Paradise Lost.
The Grim Up North in this story contrasted against the warmer locations in China and the Pacific Ocean where some of the story's more optimistic scenes occur continues a theme in King of the Monsters: barren, icy locations are associated with Ghidorah, whereas hotter and more tropical locations are associated with the likes of Mothra.
The Titans' complex Animal Talk makes more sense here than it might with animals in other settings, because in King of the Monsters, the Titans' long-range communication was said to be whale-like, and whales are capable of complex communication.
Is Mark just being a Commander Contrarian when he's initially dismissive of a glaring lead on finding Monster X in Chapter 16? Or is he hesitant due to a combination of knowing the humans and heroic Titans played right into the Many's hands the last time they took the first bait, and because he knows his own Hot-Bloodedness has caused him to make poor calls on impulse before.
San is surprised that Vivienne rescues him in Chapter 16. This isn't just because she did such a thing and he undervalues himself, it's also because his head was severed from her beforehand – Ghidorah has always disregarded and dehumanized its severed heads that gain autonomy as nothing more than shed skins which are no longer the attached heads' brothers, so he's surprised that Vivienne completely defies that mindset.
Ichi/Eldest Brother and Ni/Elder Brother's squicky plans for Vivienne later in the story make Ichi's foaming-at-the-mouth hatred of Vivienne and Ni's vocal dislike of her that they initially had before reaching this point retroactively seem like a one-sided, very dark Belligerent Sexual Tension.
All things considered, the Final Battle starting at evening and ending at dawn seems like a real case of Magic Countdown. But considering how far north Berezniki is and that this part of the story explicitly takes place in May or June, it's justifiable that the night would be literally just a few hours long: during the summer, the closer you are to the hemisphere's pole, the longer the days and shorter the nights are at that time of year.
Ghidorah and the Many (essentially the Devil and damned legions respectively) being based in Russia (initially at an abandoned Monarch outpost) is made all the more fitting by the fact that the other, official Monarch outpost in that region (which gets mentioned by number in this story) is Outpost 66. What number makes up the Number of the Beast again?
It might seem like a bit of a cosmic Contrived Coincidence that of all the trillions of complex lifeforms on Earth that Serizawa could be reincarnated into – never mind the possibility of reincarnating on another planet's biosphere, especially since Ghidorah's memories reveal that the universe has or at least had a multitude of inhabited worlds – Serizawa not only gets reincarnated as the Titan equivalent to a prince, but he also he gets reincarnated in a body that just happens to have his previous life's Platonic Life Partner present at the birth. However, considering the author's comment that Serizawa's reincarnation as Manda was probably something of a divine reward for his Heroic Sacrifice, it's possible that getting reunited with his past life's aforementioned Life-Partner was part of the package.
During the Final Battle, when you think about it, Monster X technically kills both of San's evil brothers in some form: they personally kill MaNi/Elder Brother, and then they personally finish off Ichi/Eldest Brother after other Titans have destroyed the rest of Ghidorah.
Ghidorah is represented in MonsterVerse canon and the Godzilla franchise as a whole as Godzilla's Evil Counterpart, but in AbraxasVerse, some of the details of both characters' backstories reveal something extra they have in common: a level of Parental Abandonment. The differences are that Godzilla's mother stuck around until he was old enough to fend for himself, whereas Ichi, Ni and San's biological sire didn't even do that; and Godzilla thinks highly of a Titanus Gojira who he suspects was his biological father despite the latter's lack of involvement in Godzilla's upbringing, whereas San doesn't think much of or feel anything toward his long-dead biological sire.
Monster X in its final form lacks the physical half-skull shoulders of the original incarnation, but it can be said to retain them in a metaphorical form. If the main/real single head represents the AbraxasVerse incarnation's composite singular personality, then Viv and San's ability to still speak as individual personalities instead of a singular being via Flip Personality can be said to be a representation of the half-skull shoulders which flanked the Toho Monster X's main head. Especially when you remember that Viv and San were reduced to half a head each before reaching their final form.
Given that Godzilla appears to be firm about ensuring the Titans stick to their territories when there's no threat to the world he has to focus on, Okinawa is an appropriate territory for Monster X to stick to for a couple reasons: besides the access it gives Monster X to a certain someone on Okinawa, Okinawa is also on the edge of the late Serizawa's homeland.
On a meta level, Okinawa is also on the edge of the country where Godzilla as a character and a franchise originated in 1954.
One which was contemplated by the author herself post-publication. When a reader asked if Monster X's own Psychic Powers could have contributed to the Brown Note effects that San's old head was having on Jonah and his Basement Club, the author noted that Jonah's and Sergeant Travis' respective Sanity Slippages are only shown to get really bad after Vivienne's Howl of Sorrow.
Vivienne Graham in this fic continues the MonsterVerse movies' tradition of featuring a human (Metamorphosis in Vivienne's case) protagonist whose character arc concerns the tragic death of a loved one.
It isn't just coincidence that San's head out of Ghidorah's three heads was the one charged with "safekeeping" Vivienne's body in his neck after the middle head ate her, nor is it just due to San being given the menial work. Ichi/Eldest Brother doesn't want to keep Vivienne in his own neck because that'll limit his ability to indulge his sadism, being unable to use his Gravity Beam so long as Vivienne is gestating inside his neck – and Ichi has the biggest appetite for inflicting pain and suffering out of all three heads. Ni/Elder Brother isn't a wise keeper either for a couple reasons: Ni cares the least about Ghidorah's plan for Vivienne out of the three, he'll be as frustrated as Ichi if not more at having to restrain his Gravity Beam since he's the most prominently battle-thirsty head, and his shed skin MaNi is proof that he really needs his brothers acting as his Restraining Bolt to avoid slipping into Stupid Evil; so Ni/Elder Brother overall probably can't be trusted to go out of his way to avoid accidentally atomizing the gestating Vivienne through carelessness if she was inside his neck. San however is not as deeply drenched in an endless thirst for suffering, bloodshed and domination as either of his brothers are, and he can therefore take a backseat from using his Gravity Beam in battle and slaughter, plus the fact San's genuinely interested in keeping Vivienne alive means he's unlikely to get distracted from ensuring her body remains intact and her gestation into a new Titan completes.
The details of Ghidorah's death in the Final Battle closely reflect how it died at the climax of King of the Monsters: its side-heads and then its body are annihilated in Kill It with Fire fashion, and Ichi/Eldest Brother's bodiless head afterwards gets burned to death by the signature energy weapon of a heroic Titan (who has a personal rivalry with him) being used at close range. It's almost like it's meant to emphasize that whilst Ghidorah always has a chance of coming back, its enemies also always have a chance of defeating and killing it all over again.
In Chapter 17, Monster X is holding Ichi/Eldest Brother's severed head up by the horn. It's previously been established in this fic that rough treatment of a Ghidorah head's horns such as forcibly grasping and pulling at them is roughly equivalent to physical abuse such as a violent slap or shaking among their kind – holding Ichi's head in such a way is just another, little way for Monster X to make him hurt.
In the Godzilla vs. Kong novelization, it was implied that one of the things that made Mark convince himself that Madison needed to be sent to a public school was her needing to make friends her own age. But in this fic, Sam Brody (who is only a couple years younger than Madison), is also a Monarch kid like Madison; and although he never directly interacts with Madison, his father Ford is part of the G-Team which works closely with the Monarch top brass, and he still has regular contact with him. With the above in mind, this could provide an additional reason besides the obvious for why AbraxasVerse Mark was successfully talked out of forcing Madison into an Obsessively Normal education in favor of continuing her upbringing at home and in Monarch.
Madison's expression of disgust at Emma's inaction when Jonah threatened to have Madison's throat slit if Emma defied him makes Emma seem a little Unintentionally Sympathetic in this instance... unless you look at the way Madison worded her rant, wherein she describes this incident immediately after she's described Emma's lack of empathy for the people of Isla de Mara, and you take away a different reading: Madison isn't so much chiding the fact Emma submitted to Jonah's whims to keep her daughter alive, as she is Emma's Moral Myopia in placing so much value on Madison's life but not the hundreds of other innocent lives at Isla de Mara.
Ghidorah's tissue turning into a mutated Assimilator after a few crude experiments by humans who had no idea what they were doing, and Ghidorah's implicit ability to permanently assimilate other creatures besides its three heads into itself if it chooses to; aren't just a Sci-Fi Kitchen Sink. Ghidorah itself was originally created by a forced Sibling Fusion of three captive creatures by the Makers, so the aforementioned assimilative capabilities of Ghidorah's tissue are likely either: (A) a lasting remnant of the Makers' experiments locked away in Ghidorah's DNA, or (B) a natural dormant trait of Ichi, Ni and San's original pre-fusion species which the Makers exploited to reconstruct the trio into Ghidorah.
Mark being unpopular among the larger Monarch workforce and resented for his indirect role in Vivienne's apparent death makes sense for a few reasons. (1) He'd been outside of Monarch's social circle for half a decade, and Coleman probably wasn't the only friend of Vivienne who'd only joined Monarch after Mark's departure and therefore didn't really know Mark beforehand. (2) Mark's obnoxious attitude towards everyone in Monarch even while they were trying to rescue his family for him and his professed disdain for Titans during the first half of the movie realistically wouldn't have made a good first impression on anyone. And (3) Vivienne before her seeming death had been a top-notch, high-ranking Monarch operative who was throwing herself all into the organization's work, while Mark was pushing away everyone who tried to help him in favor of doing nothing but wallow in self-pity at a mountain cabin.
Ghidorah's left head being the only one of the three that has any actual redeeming qualities, whereas both the story and Word of God have made it quite clear that Ichi/Eldest Brother and Ni/Elder Brother are completely, irredeemably too far gone from who they once were; connects quite well with the fact that the left head is the Youngest Brother. Ghidorah's heads became as evil as they did due to sharing a horrific Dark and Troubled Past, and Word of God indicates Ichi Used to Be a Sweet Kid before Ghidorah's descent into darkness turned him into the pure-evil asshole he is now. What are elder siblings that are charged with safeguarding younger siblings' welfare known to do in the face of trauma and horror? Protect and help their younger siblings as best they can. It's possible San and San-Who-Could-Have-Been/Youngest Brother retaining a piece of good in them long after Ichi and Ni's former goodness had completely faded is the result of the left head's older brothers trying to protect him from the worst of their trauma. This brings a new tragic layer to Ghidorah's descent into darkness: Ichi/Eldest Brother and Ni/Elder Brother essentially sacrificed their remaining goodness so that San could retain his, enabling San to eventually find peace and a family again whereas Ichi and Ni never will.
This also bring in a bit of Fridge Horror: if the left head really did retain some redeeming qualities because his brothers initially tried to minimize his torment before they became fully corrupted, just how bad could Vivienne have ultimately become if Ghidorah had assimilated her, with Ichi/Eldest Brother and Ni/Elder Brother actively doing the complete opposite to what their former selves did for San toward her?
As mentioned in both MonsterVerse canon materials and in Abraxas, Vivienne was born in 1976. This story sees her get resurrected as a draconic creature, and as pointed out on the author's Tumblr, '76 was the Year of the Dragon on the Chinese zodiac.
The author says that most of the human supporting characters' remaining reservations towards San for his past likely lowered even further after they witnessed Monster X fighting on Godzilla's side at Berezniki. But there's another reason why the events at Berezniki would convince them that San really is on the good guys' team now: during the battle, the humans would have also witnessed when the other San that was attached to Keizer Ghidorah chose to turn against Ichi/Eldest Brother and Ni/Elder Brother in favor of aiding Monster X and sacrificing himself.
Mark eventually accepts that Madison is too drawn to the Titans to be kept away no matter the mortal risk and no matter what he does, but he still makes her promise to stay close to him before the Final Battle, and Word of God confirms that losing Madison (which is a very real possibility for any human who gets directly involved in Titan matters) is still Mark's worst fear. Considering Mark in MonsterVerse canon considered not being there for Madison when she needed him before the Mass Awakening to be his greatest failure, it's likely that Mark's reasoning later on in Abraxas is that: rather than counter-productively attempting to cut Madison out of Monarch to keep her alive, he wants to at least make sure he's always by Maddie's side if she's really going to be hanging around Monarch with the occupational hazard that entails for the foreseeable future, and if worst comes to worst, Mark can at least honestly say that he was there for Madison this time.
Gigan, though not a direct Contrasting Sequel A Ntagonist to Ghidorah, thus far contrasts it in several ways that distinguish the two Ax-Crazy alien Titans. Both are malevolent, extremely sadistic and vicious extraterrestrial Titans which travel the stars, and as of the main fic's events they've both been worshipped as a god by an army of smaller, intelligent but twisted creatures. However, whereas Ghidorah is an Omnicidal Maniac that lives firstly for killing, and it especially seeks to wipe out any sapient "little ones" which are capable of building technology; Gigan by all accounts lives more for making his victims suffer than anything else, and he's happy to keep an army of technological "little one" sense freaks alive as his thralls and worshippers. Whereas Ghidorah was an organic, biologically regenerating Draconic Abomination, Gigan is a Cyborg.
Did Ghidorah manage to exterminate all of its Makers, or did it kill only a group of them? The author herself has suggested that part of the reason Ghidorah originally became a Planet Destroyer beyond satisfying the Old Noise was because it discovered colonies of the Makers' species were still alive on other worlds.
Were these other Makers that Ghidorah exterminated more benign than the ones who made Ghidorah?
If they were just as bad as Ghidorah's creators, could Ghidorah have still failed to catch and wipe out every extraplanetary colony?
It's confirmed by San's dialogue that what happens in this fic following Ghidorah's death in Boston isn't the first time Ghidorah has died and then come back From a Single Cell at all. San-Who-Could-Have-Been/Youngest Brother himself brings up the hypothetical possibility of two Ghidorahs existing at the same time if Ghidorah's main body remains alive whilst a shed skin is regrowing its own lost biomass. And we know Ghidorah is a Planet Destroyer that has been around for billions of years. What are the odds that Ghidorah left shed skins behind on previous worlds it destroyed that put up a fight, and that these shed skins regrew into Ghidorah clones or into new hybridized Children of the World Eater like Monster X? Which means there could very well be more versions of Ghidorah or more Ghidorah-derived "children" still out there amongst the stars, continuing Ghidorah's omnicidal campaign on other worlds. The author herselfaddresses this.
It's not mentioned in the story but it's entirely possible that children were among the Disposable Vagrants whom Jonah and his Basement Club turned into the Many, especially since Jonah Would Hurt a Child. A Deleted Scene would have confirmed that the Basement Club were at the very least setting their sights on potential child test subjects before Vivienne scared them off.
The 1920s Soviet-built outpost was studying the chuchuna that were linked to Thor, dissecting at least one specimen. Considering there's nothing to suggest there are any chuchuna still alive in the present day, it's possible the outpost's original staff caused the chuchuna's extinction whilst collecting specimens, not unlike how the dodo or other animals went extinct due to human overexploitation and lack of conscious concern for preservation.
This story reveals that Ghidorah's heads literally lack a concept of lying, which makes it seem like they can't or won't ever use outright lies in this story – until in Chapter 12, when Ichi/Eldest Brother's voice tells Vivienne, "YOU'REKILLINGYOUROWNMOTHERYOU'REKILLINGHERYOUKILLEDHER". This seems to hint that whereas San considers dishonesty to be a waste of mental power once he wraps his head around the concept, Ichi and Ni after gaining humanity's collective socio-cultural knowledge can and will lie to get the desired effect. Word of God discussed this and confirmed that whilst none of Ghidorah's heads outright lie, Ichi/Eldest Brother is all too happy to use Exact Words to manipulate others.
It's confirmed in Chapter 17 that the Many have discovered a way down to the Hollow Earth (specifically the fertile inner realm that was portrayed in Godzilla vs. Kong). Even after the Final Battle, if the Many are still down there or remnants of the Many still know the way there, could they invade and assimilate the Hollow Earth's ecosystem, possibly even target the Zohar and complete Keizer Ghidorah's plan of physically killing the planet from the core? Note that the final chapter hints that not all of the Many are dead or inactive at the story's end.
Ghidorah was said to be weakened and using the Many to compensate in the final battle against Godzilla, Mothra and six other Titans. If that's how a battle involving that many heroic Titans including the King of the Monsters himself versus a lone and weakened Ghidorah went, how would it have gone if Ghidorah had been fully restored?
Remember all of Jonah's mercs whom Viv and San killed by dismemberment? Since they didn’t suffer Fire Keeps It Dead by Viv and San's lightning like the final Mook Horror Show did, what are the odds that after their remains were cleaned up, they got turned by Jonah's Basement Club into some of the Many? (Granted, their deeds in life would make them Asshole Victims if this is the case, but still.)
Monster X makes regular visits to Susan during her stay at a hospital until her death. Considering Monster X's powers' electromagnetic properties which have caused Walking Techbane in the past, nevermind how their EVP voices can affect radio devices without them even trying (Ren Serizawa even mentally brings up the possibility that Monster X's EVP can do lasting damage to the affected hardware); what are the odds that every time Monster X visits the hospital, they could accidentally interfere with hospital equipment that's being used to perform critical and life-saving operations at the time? Never mind how the vibrations caused by their giant footsteps could cause surgeons to slip up in the middle of critical surgeries or otherwise force them to stop operating.
It's confirmed that Ghidorah has killed many inhabited worlds during its eons-long campaign of inter-planetary conquests and destruction. It's quite possible that Ghidorah's intergalactic omnicidal campaign before coming to Earth is the main reason why humanity hasn't discovered any evidence of other complex extraterrestrial lifeforms or intelligences despite advances in astronomy and S.E.T.I..
Skull Island is Spared by the Adaptation, but Kong still has to leave once he's fully grown due to technically being an invasive species that the ecosystem can't sustain anymore. But what'll happen with the Skullcrawlers invading the island once Kong isn't around to hold them back anymore, assuming that the island's Vile Vortex (which was sealed in the battle) isn't the Crawlers' only vent up from the Hollow Earth? What'll stop the Skullcrawlers from proliferating out of control, annihilating the Iwis and the island's ecosystem, and heading for the mainlands as Dr. Brooks feared they could in Kong: Skull Island without Kong?
Since Ghidorah is composed of elements that aren't on the periodic table and since Outpost 32 were studying Ghidorah's genetic samplesnote Mentioned briefly by Vivienne in the movie, why didn't Monarch discover early on like Jonah's paramilitary did that this indicates Ghidorah isn't originally from our planet at all?
The answer is, perhaps Outpost 32 did become aware of this, but Vivienne kept the knowledge to herself when human due to the frozen Titan's The Dreaded status.
San mentions twice in the story that he's never had a sister before Vivienne, and he mentions in the first chapter that he's never been a single being without another attached to him before as his severed head temporarily was. This doesn't quite match up with Ghidorah's Massive Numbered Siblings in its backstory which are described by San as consisting of both brothers and sisters, and it also doesn't go fluidly with the revelation that Ghidorah has died and regenerated From a Single Cell before.
Ichi, Ni and San's and their other siblings' original birth occurred so far back that San himself can scarcely recall most of the details anymore. Another reason why San might consider Vivienne the first and only sister he's ever had is because she didn't die moments after their birth like his and his brothers' other sisters did.
As for the bit about San being alone as a single being; it's possible that while Ghidorah has died and only left behind pieces of itself before, this is the first time that that From a Single Cell piece ended up being San's severed head instead of one of the other two heads or another body-part.
Why does San recall Ghidorah roosting on Rodan's volcano in Chapter 13, seeing as this was after San's decapitation?
Perhaps San heard the call from the ocean floor due to Titans' whale-communication and was able to pinpoint Ghidorah's location from the call.
A reader questioned here why Viv and San's breathing wasn't permanently crippled by their Healing Factor's Fibrodysplasia ossificans progressiva defect when it healed their lungs from chlorine. The author admitted she hadn't considered that before providing a possible answer.
It's ultimately uncertain whether Monster X is truly The Ageless like Ghidorah, or due to the human hybridization is merely Long-Lived (similar to how the hybridization causes Monster X's Healing Factor to work differently from Ghidorah's). If the former is so, Monster X will have to watch not only all of Vivienne's human loved ones from her old life die one by one, but will likely end up outliving everyone else they ever grow to care for except one-another, even the other Titans, unless Monster X is felled in a fight to the death by another Titan in the distant future. Cue Who Wants to Live Forever?.
The author suggested that MaNi/Elder Brother's remains could be one of the ways Ghidorah can hypothetically come Back from the Dead again after the story's ending – since MaNi became autonomous from Ghidorah long before Monster X and Thor instigated San-Who-Could-Have-Been/Youngest Brother to turn against the rest of Ghidorah (and MaNi furthermore did not witness San-2's turn in any capacity), a Youngest Brother/left head regrown from MaNi's remains will have no memory of San-Who-Could-Have-Been's actions and will be just as subservient to Ichi/Eldest Brother and Ni/Elder Brother as San and San-Who-Could-Have-Been originally were. Doubles as Fridge Brilliance because it would also be a useful plot device to explain how Ghidorah can return from the dead without its threat level being severely hampered by a constant open war between its heads if any Fan Sequels explore Ghidorah's next resurgence.
about how Monster X's silver theme is so appropriate in the story beyond the Silver Bullet trope.