The Servant of the Flame is the representative of the Reaper's Bones trading company, based out of the Reaper's Hideout on the Sea of Thieves

The Servant of the Flame's identity, history, and employer were previously a mystery.
Given the connection the Masked Stranger's involvement with The Reaper's Hideout, it was assumed that the "Flame" they serve is Captain Flameheart, whose stance on piracy bears semblance to the sentiment behind the Reaper's Bones company.