I'll just cut to the chase immediately: Esmy is The Lost Kin
I also have reason to believe that Esmy is COMPLETELY separate from the Nail Brother Beef, none of them mention it, not even in their minds, and it's obviously a red herring as to who Esmy is.
Anyways, I'll lay out the basics:
The Lost Kin obviously knows Nail arts, in case you forgot the fight, they use an imitation of the dash slash (they simply dash and use a great slash, presumably due to being a REANIMATION of the Lost Kin) which implies that they know both of Oro and Sheo's techniques, however I also want to note their side to side sword slash they perform, I don't think it's an imitation of cyclone slash, it actually brings me to the next part of this theory
In relation to Sly, I have reason to suspect a mentorship, as Sly is the only other person that could teach a person of the Nail arts without the Brothers. not only that, but also the side to side slash move they do during their fight look incredibly similar to a move that Sly performs during the Fight of The Great Nailsage, I suspect this a Nail art that was purposefully kept from The Ghost out of respect for the deceased.
I also believe this could open the possibility of what happened to the Lost Kin, unlike you or other vessels when they die, the Lost Kin has no Shade, rather they're a hollow corpse with a cracked open head, one of my assumptions gathered around a specific scenario: Esmy was bringing Sly (their mentor and a person they would trust) down to ancient basin, but before they could go any deeper, Esmy was possessed by The Light due to a similar idea instilled to the Knight (their mentorship) due to this infection, Sly had to old yeller Esmy in gladiatorial combat, and if you've completed the Pantheon of The Sage, it's understandable as to why Esmy lost.
On ANOTHER side tangent, this theory would also tie into the Vessel Growth theory, as The Lost Kin is notably taller than the Knight (not as tall as my goat tho), as Sly's mentorship could've trained them to proper growth