Frisk's appearance is never seen throughout the Penumbra series, prior to and after his transformation. However, it is widely believed that the infection turned him into a spider-like creature afraid of light and capable of extreme agility, allowing him to over-power any prey he seeks, how he can attack a victim in the dark without them seeing him.
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.
Because of the theory that Wilbur Frisk may have the body of the Hunter. It is possible that the Hunter that stalks Phillip throughout Penumbra: Necrologue may indeed be Wilbur Frisk. This is further supported by the Hunter making similar noises to that of Frisk in Penumbra: Black Plague. In Black Plague, Frisk needs heavy darkness to attack Phillip, reflecting this behavior, almost every area Phillip encounters the Hunter in are extremely dark.
Clarence also implies that it's indeed the same Hunter that keeps reappearing throughout the game and only one Hunter is ever encountered at one time (Frisks mutation was reported as a unique variant). Additionally the same burrow holes seen in the kennel where Frisk stalks Phillip appears in areas where Phillip encounters the Hunter.