I guess since I already talked about Nyleth I could talk about Seth as well
The fluffy coat over his neck seems to me like a cloak made out of shellwood leaves, sharing that greenish grey color
I think this coat granted him longevity and strength in exchange for a trance-like state in servitude to his role as guardian, since he was "chosen by the voice (Nyleth)" for said role
And when he loses this coat after the fight, it takes his memories with it, which might remind one of Quirrel or Nymm greatly; but unlike Quirrel, I think the leafs were actively nurturing him rather than just keeping a stasis, explaining why Seth maintains that youthful energy even without it
He's definitely older than the Haunting, and potentially as old as the foundation of the Citadel
There's the deal between the Citadel and Nyleth, but also the shrine and the path to it are built on crude stone bricks and pillars, an architectural style visibly older than any of the steam pipes and polished, gilded buildings of modern citadel
His pin is a weapon characteristic of the Citadel, but his cymbal...
Cymbals seem to be a symbol of the pilgrims and their pilgrimage, acting as plates for offerings of rosaries, present in stonework and tolls alike, the later likely evolved from said statues and offering shrines
However, outside of Seth, we only see a few bugs carrying a cymbal
Sherma does hold quite some similarity to Seth, specially when you consider that sherma has his head wrapped in cloth
so Seth and Sherma might be of the same species, maybe even from the same origin
Considering all this, Seth was most likely a pilgrim "armed" with a cymbal, carrying it as a custom of faith, either for offering rosaries or music or both, which he then had to use as shield and weapon once chosen as guardian
and at some point during his long story of guarding the shrine he picked on a pin, either by pinstress training or from the corpse of a felled member of the choir, maybe even learning from his adversaries as they came
he possesses moves similar to that of second sentinel, which in turn are at least partially learned from a pinstress; however, he does not employ the cross edged pin strike characteristic of both, so I'm gonna lean into the second idea of learning from opponents