While we're at it, do you have any headcanons about the Agarthans in general? Or maybe some specifics for the other named ones?
Headcanons for 👾Those Who Dubstep Deep Below👾? Certainly. This'll be a long one.
(Reminder that these are all headcanons and not intended to be fact.)
My personal theory is that their numbers dwindled so much that they simply decided to stop having children altogether, instead focusing their energy on science and prolonging their lives to the same lengths as Nabateans using what they learn from their corpses.
The result of their research took the form of vessels that could be made and remade over and over again to implant themselves into - the exact same thing Rhea was doing to bring back Sothis, only much more successful. It is why, despite apparently being the original humans, you have Agarthans that have existed since the fall of Nemesis when they shouldn't be able to live this long.*
*Side note: If you want to think of it in an Adam and Eve situation, the Agarthans could already have immensely long life spans as the first humans, meaning the humans of Fodlan created by Sothis have much shorter lifespans as a result. (Though this could easily be debunked by looking at characters from other countries.)
Though, I would say there are outliers like Kronya, who is very obviously younger than the rest of them. She likely was one of the few new Agarthans, likely born within 100 years or less of the current timeline. So, maybe they can have children and occasionally do, but in a Frieren elves situation, they have no desire to.
He is the best at what he does, but not because he's been doing it the longest. Solon is simply the most proficient at it. Solon probably took to body snatching and the research required to make proper hosts much faster than the rest, clearly showing a much higher capacity for deception and cunning than the others by moving about undetected throughout the entirety of his time as Tomas.
And I believe it is because he worked with Epimenedes directly, but was swayed by Thales to betray him.
Experiments on himself using magic/tech he's already tested on other disposable subjects. (Probably why he appears more monstrous than the others.)
Had a hand in the construction of magic cores and direct access to Nabatean corpses.
Probably part of the group that experimented on Lysithea (would explain how he learned about Tomas, who came from Ordelia)
Very obviously young by Agarthan standards. She hadn't had enough time to learn the long-con, so people were immediately suspicious of her. In addition, her begging Byleth to save her life in 3Houses when Solon used her as a sacrifice also speaks to her age. Were she older, she never would have considered entreating a "lowly beast" to help her, even if she was dying a slow and agonizing death for the sake of her people's future.
If I recall correctly, none of the other Agarthans in 3Houses/3Hopes ever did that. Not even once.
Nepo-baby. Only working in the open because she'd otherwise be in the way in Shambala.
Collects trophies from missions.
A fantastic fight, but "she jinxes every job."
Keeps getting herself into life-threatening trouble that results in her body needing to be replaced too often.
Young enough to not know who Epimenedes is.
Could potentially have been successfully rehabilitated.
(I have quite a bit about this guy, actually)
Second only to Solon in terms of intellect, leaning more towards constructs than organic experiments, but 10x as cunning. His charisma is like that of a devil striking a deal, bringing out the worst in you and waiting until you've sunk so incredibly low that your only way out is taking his hand, essentially feeding off your pain and misery without you even noticing.
It's probably why he took to wearing the body of a woman so easily. I think Cornelia herself didn't dress as extravagantly as Cleobulus did while using her body. If I had to guess why, it was strictly because he wanted to appeal to as large an number of people as possible using the *ahem* "tools" he had available.
He was masquerading as a bombshell of a woman in one of the coldest places in Fodlan, all while wearing a high slit dress and bare shoulders, putting aaaaall the "merchandise" on display for the world to see. There was... no support in the bodice of that dress, and it was absolutely on purpose. Considering how the other women of Faerghus dress with a fair amount of modesty and practicality in mind, this was probably incredibly out of the ordinary and very enticing. Honeyed words to mask the sting and the body of a supermodel. He had the method down to a science. Unfortunately for him, a few people close to the throne were immune to his wiles and questioned him a lot.
Impeccable fashion sense and hygiene routine, even in his original body. Made the upkeep on Cornelia's body that much easier.
Much younger than Thales, much older than Kronya.
Probably had suitors as I Can't Believe It's Not Cornelia™️, and all of them "mysteriously" went missing.
Absolutely wears the "recently widowed" silk sleep set around the house, imported from Enbarr. You know the one.
"I have a heart. A few, in fact. They're in a jar up on my bookshelf."
Myson probably worked alongside Solon a lot, likely putting him in the direct line of contact with Epimenedes in the process. Judging by his reaction to seeing Shez using that power, he probably never expected the guy to come back, let alone have a new body to use. Yet another rat, even if he did have roughly the same ideals as Epimenedes.
Old and tired, but too busy to die.
Charisma stat in the negatives and still rolls a Nat20 when it counts. "Well, that was easy."
Probably involved in the experiments on Lysithea.
Treats Kronya like an annoying younger sister when in Shambala, but they're the personification of the "when two dumb girls say "Exaaaaactly"" meme when they work together.