The Forest Folk are an intelligent, deeply religious, and nearly forgotten humanoid race, living in the Yggdrasil Labyrinth of Etrian Odyssey. They live in the Sandy Barrens, and, for some reason, are unwilling to let the explorers go any deeper into the labyrinth, citing an ancient pact with the humans of Etria, and will do anything in their power to stop anyone from entering the depths of their lair.
The Forest Folk are depicted as slender, charismatic humanoids with greenish-toned skin and hair that distinctly resembles leaves, often brightly colored. Protracted exposure to the Yggdrasil Core and experiments by Visil have caused many of their number to mutate grotesquely, turning entirely into monsters. The Forest Folk are matriarchal, with their priestesses and shamans all being female, with males generally being warriors, hunters, and guardians.
The most well-known member of their race is Kupala. In the remake, their origins have been fleshed out more. Kupala is revealed to be their priestess and Alraune was once their queen before being mutated into her current form.
The Forest Folk were not very fleshed out in the original game, but over the course of the other games and remake in Untold, were given a much more in-depth backstory.
At peace in the Labyrinth for generations, the Forest Folk are believed to once have been an engineered race, much like the Yggdroids of the Deep City, the Birdfolk of the Petal Bridge, the Vessels of the Misty Ravine, and the Sentinels of the Golden Lair - this is brought about by multiple bits of information in EOIII, IV, and Untold. Like the Vessels (whom, in the Japanese version, are called the very same species name, Mystics), they were engineered for the purposes of helping humanity rebuild after the apocalypse. In this case, the Forest Folk were intended to help dispose of the core when the time was right.
Kupala's interactions with the party in Etrian Odyssey Untold indicate that the Forest Folk hold a deeply religious belief about their role in the Labyrinth, and see their duties as sacrosanct. In a bygone age, the Forest Folk made a pact with Visil to control the Yggdrasil Core, and, when the time was right, destroy it. As Visil lost his humanity, he began to find ways to deal with the Forest Folk, unleashing a plague upon them and attempting to hire adventurers to engage in wholesale genocide of their race.
The party of Etrian Odyssey first encounters Kupala in the third stratum, where she delivers dire portents about adventuring further into the labyrinth, and cites the ancient pact. When the party claims ignorance, she is incensed, believing them to be liars and decievers. Soonafter, Visil, obsessed with getting rid of the Forest Folk due to their opposition of the core, begins hiring adventurers to exterminate the Forest Folk. In the original, the group is left with no choice but to indulge in this genocide on Visil's behalf, but in Untold, they further look into it, trying to find out if there is a better way, and quickly learn that Kupala's apprehension stems from many of her race going gradually insane; Though the Highlander and his party, with help from Frederica Irving, consult M.I.K.E. about the infection in the hopes of finding a way to cure them, they quickly learn that the virus is incurable once contracted and the only way to stop it is to kill those infected before they can infect others. They try to talk Kupala down again, but ultimately fail, and she unleashes Corotrangul on them fleeing when the group destroys it.
By the time the group encounters Forest Folk that aren't Kupala, in the Sandy Barrens, the civilization of the Forest Folk is on the verge of tearing itself apart. A malady is spreading through their numbers, turning them murderous and cruel, tied to a viral outbreak that stems from the Yggdrasil Core. Kupala is unwilling to listen to the group's pleas - she either falls in battle (Etrian Untold: Classic Mode, or Etrian I) or is rendered unconscious and brought back to the guildhouse (Etrian Untold: Story Mode). The Forest Folk are left to their fate in the case of the former; in the latter, Kupala eventually sees the reason behind the group's actions and elects to assist them, explaining to the group the history and beliefs of the Forest Folk, and her ultimate role in helping to send off their god when it dies. The group later learns that this means Kupala intends to assist them against the Yggdrasil Core; she ultimately martyrs herself in order to empower their weapons against the beast.
Canonically, the Forest Folk survive Visil's attempted genocide of their species, and they eventually make peace with the humans of Etria. A member of their forces, Lindworm, comes to Armoroad to beseech the party's aid against the Storm Emperor in Etrian Odyssey III.