You're right about the last sentence. Earlier this week, the Brazilian forums were hit up by a Riot account named Fabulista (fable-teller), which was the same account used to narrate the story of River Spirit Nami. They chatted with forum-goers and even answered some questions, but the responses have grown increasingly hostile and erratic over the days, with the character acting as if possessed and switching from describing themselves with masculine pronouns to a more sinister "we".
You can find the translations of their forum posts here, here and here.
I'd like to especially point out the following lines:
"There is no "Fabulista", only the mist".
"Who I am does not matter. Who are WE? We are the fallen. United as One, we will ravage those who are against Us--and Our forces only increase."
This is only speculation on my part, but it's likely that Kalista isn't a specific man or woman. Instead, it's an amalgamation of many souls who were killed wrongfully in the past, and used the power of the Shadow Isles to unite into a singular powerful being in their quest for vengeance.
It's also very likely that it's not allied with Shadow Isles per se, since one of its seven targets seems to be
despite claiming that "The Paladin of Pain and Cleric of Death march to the same beat of the drum as We". We already know from both pre and post-reboot lore that
and
have their own agenda separate from the Ruined King or whoever it is that rules over Shadow Isles.