Wilbur Frisk was the Chief Overseer of The Shelter, although he only made a minor appearance in Penumbra: Overture and was only referenced in the note at the end of the game, he is referenced numerously throughout Penumbra: Black Plague and even appearins as an unseen enemy that Philip encounters.
Frisk's appearance is never seen throughout the Penumbra series, prior to and after his transformation. However, post-transformation he is found in the kennels displaying extreme agility and heightened senses, being able to easily overpower his prey without being seen in the dark. Due to this, it is believed that he transformed into a creature similar to the Hunter, a cut enemy from the Penumbra tech demo, as Eloff Carpenter writes in his journal entries about Frisk's unnatural new form, Frisk digging and hiding in tunnels that lead to other locations, and improved senses, such as Frisk being able to stay out of Philip's sight when Philip enters the kennels allowing Frisk to easily overpower and kill him, are all abilities that the Hunter's abnormal form could provide.
Most notably is that the glowing white eyes of the Hunter, which would likely work as natural light in dark areas, would be extremely sensitive to other light sources. This could explain why Frisk is afraid of light and only roams dark areas of the kennels, as the bright industrial lights of the kennels would be blinding to him, making him vulnerable.
Elevated Caste member Eloff Carpenter began a report on Frisk on February 20th, 2000 after Frisk turned up few results on his projects since taking over as Chief Overseer after Bren Stafferson. After the Tuurngait Virus attacked the station, he escaped to the Kennels for protection from the dogs, with Eloff following him to continue his report.
During the weeks that passed, Eloff notes in his report that Frisk is becoming more friendly with the dogs, and even begins to show some primal instincts as if reverting to them, one of the results being digging out tunnels, most of which lead all the way back to the old mines. These tunnels may have been the cause to the dogs escaping to the old mines, as no dogs except a few corpses can be found in the Kennels.
Eloff notes that, despite having rations, Frisk starts eating some of the dogs, possibly the cause for Frisk's transformation.
In Penumbra: Overture, Philip witnesses Frisk attacking Dr. Roberts before killing him and dragging him away to the Kennels, as evidenced by Eloff noting in his report that he woke up to find a corpse missing its tongue lying next to him, the man having escaped to the old mines shortly after the virus outbreak.
The last Philip sees of Frisk is a document found at the entrance of The Shelter listing Frisk as chief overseer.
In Penumbra: Black Plague, by the time that Phillip enters the Kennels, Frisk has completely transformed into his spider-like form. He will attack Philip in the darkened hallways by coming out of the holes he has created, and can easily kill Philip. However, due to to his sensitivity to light, Philip is safe if standing in an illuminated area, and Frisk will not leave the Kennels.
Eloff's reports on Frisk can be found in the electrical room of the Kennels. Judging from the claw and bite marks, along with the location of the corpse, it seems Frisk killed the scientist holding the research room keycard at the end of the Kennels.
- Frisk appears in the fan full-conversion mod for Amnesia: The Dark Descent, Penumbra: Necrologue, even using the Hunter model from the tech demo. Not only does he use similar sounds and behavior from Penumbra: Black Plague, Clarence also implies that it's indeed the same creature that keeps reappearing throughout the game, with only one Hunter ever encountered at one time and Frisk's mutation being reported as a unique variant. Additionally, the same burrow holes made by Frisk in the kennels appear in areas where Philip encounters Frisk.