The Alchemist

The Alchemist is an unseen character on the island.

He intends to develop a cure for the Malaise, though the efforts are so far unsuccessful.

While not supported by direct evidence, it is heavily implied that The Collector, the true final boss of the game, who is is also trying to create a cure, is actually him.

Lore
It was implied in the description of the Guardian's Haven that the Alchemist was rather unpopular, or rumored to be so due to his unknown actions.

"The Giant didn't like the Hand of the King, and the Hand of the King didn't like the Giant. But they did agree on one point: nobody liked the Alchemist."

Some soldiers sided with the Alchemist against the King. In the Forgotten Sepulcher, one of those soldiers, who was in direct contact with the Alchemist, built a secret lab, presumably to do experiments related to the Malaise."'I realize the risk I'm taking by writing this letter. The King's methods are getting us nowhere and the situation is deteriorating. We have to switch a more peaceful method immediately! I've spoken with the alchem...' — Letter found next to a clandestine lab"

Sewers network
In the Toxic Sewers and Ancient Sewers, the Alchemist was collecting mold and mushroom samples for his experiments. However, the increase in the numbers of revenants made his work increasingly difficult. "'It is becoming increasingly difficult to collect mold samples in these sewers. There are too many revenants.'"

Promenade of the Condemned
In the Promenade of the Condemned: "'All species found in these areas seem changed. Was it the insects that spread the Malaise all over the island?'" The Beheaded mentions that the Alchemist was very "thorough" upon inspecting the insects next to this grimoire.

Ossuary
The Alchemist was applying a substance found in the Sanctuary on bones from corpses, likely executed prisoners. The substance's effects were unpredictable and the Alchemist reflects whether his experiments are worthy or mass murders from the king are more effective. "'The sanctuary substance produces unpredictable results on the bones I've collected here. All these bodies... All these lives. At the end of the day perhaps The King is right?'" About the bodies next to the grimoire, the Beheaded mentions it's: "'Hard to say whether these bodies were infected. But one thing is for sure: their skin melted like old cheese in the sun.'"

Slumbering Sanctuary
The Sanctuary seems to have been especially important to the Alchemist's research. It's the only (?) biome that contains two different Grimoires.

The Alchemist writes that people believe the sap flowing through the walls of the Sanctuary is responsible for the Malaise, by contaminating the sewer network. He also suggests that this same substance could be the solution to the epidemic, though he has never really confirmed it. What is sure is that the Alchemist harvested and used this substance as part of a treatment for the infection, as indicated by the Grimoires of the Ossuary and the Clock Tower. "'Some say the sap that runs through the walls is flowing into the sewer network and causing the Malaise. But perhaps the problem can also be the solution?'" In another Grimoire, the Alchemist references the Time Keeper's efforts to combat the Malaise. He implies that she is manipulating time to contain the Malaise, but that her spell cannot contain it forever. "'Time is running out... literally. With all due respect, she cannot contain the Malaise forever'"

Clock Tower
In the Clock Tower and the Castle, a grimoire reports that the Alchemist found a formulation that slows the down the mutagenic effects of the Malaise. "'The bodies immersed in the latest solution are changing less quickly. I must extract the essence of this solution and apply it to other volunteers'"

Forgotten Sepulcher
The Alchemist was also researching the Malaise in the Forgotten Sepulcher, but the bodies kept waking up and the Darkness was making his work difficult, prompting him to leave this area. "'The bodies have woken up again. And this darkness... I can no longer continue my experiments on the Malaise here.'"

High Peak Castle
The Alchemist was attempting to cross human bodies with plant essences, which gave rise to the hybrid found in the Castle's green Elite Room.

Cavern
The Alchemist was using the crystals mined in the cavern in his experiments to slow the Malaise's infection, as evidenced by a body immersed in a crystal solution next to his grimoire. "'We can do more with those crystals than sell them, I'm sure of that. Using them on corpses doesn't do anything for now, but I have to keep going!'"