Unknown Specimen 4, referred to as Tirsiak in the game files, is one of the specimens encountered in the Endless Mode.
Tirsiak is one of the most human-like specimens seen in the game, and resembles a young woman with amber-colored eyes and short white hair which obscures her right eye. She also has the ears and tail of a wolf, which are the same color as her hair. She has a pair of brown deer antlers which seem to have a wooden texture, and a necklace with various gold beads on it. She wears a brown top with a shawl and skirt both made of dark brown fur, brown arm warmers lined with cream colored fur, and brown leggings with a band tied around the right knee. Tirsiak is also barefoot.
Tirsiak's arrival is marked by blue mist throughout the rooms, and the silhouettes of wild animals (specifically, moose, bears and wolves) which will appear to block the protagonist's path. The animals must be destroyed with the axe in order to get past them.
Tirsiak will do about 45 damage upon contact. She goes through walls and obstacles, and is unaffected by the axe. Whenever Tirsiak damages the protagonist, she will float backwards.
If she depletes the protagonist's health bar completely, they will be sent to a snowy, wooded area. Soon after, wolves will appear from the distance and charge at them, damaging them and turning their vision redder and redder.
When the protagonist eventually dies, the camera will cut to their bloody remains scattered in the snow.
"Fur Oshus", Unknown Specimen 4's chase theme:
Unknown Specimen 4's sounds when the protagonist is near:
"Such strange prey you are."
"Let the cold consume you!"
"Death is near!"
"Why do you run?"
After hitting the player:
Unknown Specimen 4's death screen sound:
- Tirsiak's quote "Why do you run?" and Specimen 8's quote "Why do you run, child?" are similar.
- They also both feature deer antlers despite Tirsiak being based off a wolf and are both able to control animals.
- She is a reference to several Eskimo/Inuit folktales such as the Fox Woman.
- The reason for her non-frightening appearance is simply because Akuma Kira was tired of drawing creepy things, so he decided to make something cute.
- In the error messages, Tirsiak is referred to as "Tiri".
- Tirsiak's theme "Fur Oshus" was created by Jarren Christ, who also made the End Credits theme and the Spooky Kart Minigame theme.
Tirsiak now has a full 3D model and a light blue glow around herself.
Unknown Specimen 4 now has her own starting room, consisting of a snowy forest with cobble walls (similar to Room 551). A cabin stands in the middle between the entrance and exit of this room. As the player approaches the cabin, the screen will turn blue. If the player walks by the cabin, Tirsiak will spawn at the entrance door of the room and the chase will begin.
The shadow animals are larger and take a single hit to kill. If cobwebs spawn next to one of the shadows, they can both be destroyed with a single weapon swing.
A dim starry blue screen overlay replaces the light blue mist that was originally present. She will no longer float back when she attacks the protagonist; this is replaced with an attack delay of a couple of seconds.