The true identity of Ashinohara's threnodian
With the release of Hiyuki and Chisa's quests, we've gotten a bunch of crumbs about Ashinohara, and it might be enough to piece together what the threnodian of Ashinohara represents.
To start, as we've seen from Jinzhou and Rinascita, the threnodian seems to embody the main principles held by the nation in which it resides. From Chisa's story, we've seen that they're hostile towards resonators and seem to be pretty conservative, judging by being a walled-off island. On top of this, Hiyuki's story shows that they also rely more on traditions like the Mikos and prayers to battle against the lament.
Secondly, Nivora and Hiyukis dialogue strongly implies that what happened to Honami isn't some random wave-worn phenomenon and was caused by the threnodian itself. The waveworn phenomenon in Honami traps the whole city and its inhabitants in a timeloop, which boils down to being trapped in the past. At the end of the 3.3 Main quest, we find out from Nivora that Hiyuki's hometown has also disappeared, similarly to Honami, implying it's being affected by the same phenomenon.
Lastly, as we've seen especially well in Rinascita, the sentinel of a region tends to embody a concept opposing the one embodied by the threnodian. We don't know much about the sentinel of Ashinohara, but we know that the Suzu is one of the 4 gifts from the sentinel, and it has the power to manipulate futures and potential, which perfectly mirrors the threnodian having powers related to the past.
So, I think it's pretty safe to assume Ashinohara's threnodian embodies the concept of the past and stagnation.
Save Ashinohara, Kuchiba Chisa