Jeff is a humanoid zombie-likecreature that appears to be the corpse of a Combine Hazmat Worker infected by an unknown parasitic fungus-like species from Xen. The human portion of its head is split open and with it has two mouths, two protrusions that give the appearance of a Venus Flytrap (similar to those of the "Snappers," small blue Xen flora/fungus-like growths that are found in the same area) and a smaller mouth with what seems to be rows of sharp teeth. Jeff's body is also greatly swollen and disfigured: numerous lumps and spikes can be found around his body, and his right arm is much bulkier than the left.
Jeff is extremely hostile, as he will immediately try and kill the player as soon they reveal themselves within his range. Jeff also appears to be hostile to other Xen species, as he can be seen killing Headcrabs throughout chapter 7. He attacks by vomiting a dark, toxic substance.
Jeff can spew out cloud of spores from his body. These clouds can cause Alyx to cough which will alert Jeff to her presence, unless a gas mask is equipped or if the player covers their mouth.
Jeff does not take damage from any weapons. He can only be killed by being lured into a trash compactor at the end of Chapter 7.
Behavior and skills
As described by Larry, Jeff is completely blind and can't see, relying on his excellent hearing for orientation. He is agitated by loud noises and will immediately move to the source. Jeff also possesses a great amount of strength, being able to bend metal doors, and is completely invincible, only vulnerable to scripted circumstances.
Due to his status as the corpse of a Combine Hazmat Worker, he is able to pass Combine force fields.
Jeff is blind, so staying close without contact will not provoke him.
It is imperative to cover your mouth or wear a respirator mask and remove other noise-generating sources so Jeff cannot respond.
Behind the scenes
Chapter 7: Jeff was in development for three out of Half-Life: Alyx's nearly four years of development. The idea behind Jeff was to create a blind, unkillable enemy that a player would have to get creative to avoid in virtual reality. Jeff began as a Standard Zombie model and the level was built around it as an imposing entity in the player's personal space. The level's elevator sequence was part of the earliest tests to see how a player would respond to being in a confined space with the monster, to which playtesters responded so well that the moment stuck. At one point the team settled on making Jeff a giant blind Combine robot, as previous versions of Chapter 7 included a strong Combine presence. However, the intermittent firefights while dealing with Jeff affected the tension of the level, so the action elements were shed to make Chapter 7 purely a survival horror section, and Jeff was re-imagined to take inspiration from the series' Xen elements.
Jeff was designed to contrast with the art style in the rest of Half-Life: Alyx in order to signal to the player they would have to think differently for this level. Early tests included more interactions with Jeff, such as hanging objects on him or pulling a keycard out of his pocket. However, the team soon realized the sheer discomfort of being near Jeff was effective enough and so the focus was shifted to making the player do things they really did not want to do but knew they had to in order to progress through the level. The best example of this was the freezer puzzle, where the player traps Jeff but then has to let him out in order to get back into the freezer to turn the power back on: playtesters were heard cursing at the developers or wailing in frustration at the realization they had to release him. However, the developers also had to strike the correct balance for the level's difficulty: if the tension was too high or Alyx was killed too many times resulting in a game over, players would grow numb to Jeff's threat and not want to continue playing.
The spores that make Alyx cough were only added towards the end of development: playtests revealed some players became comfortable being near Jeff so long as they stayed quiet and did not touch him. Thus, the added mechanic of Alyx coughing when near the Xen spores made Jeff a more dangerous character. Once implemented, the developers observed that playtesters instinctively covered their mouths to stop the coughing. The team added covering your mouth as an easter egg, but were surprised to see playtesters discover it quickly and use it as a core mechanic, some even playing the whole level with their hand over their mouth: this led to an added line of dialogue for Larry specifically telling Alyx to cover her mouth, as the player was now almost guaranteed to do it anyway.
Jeff's death went through some different iterations: there was an understanding among the developers that after dealing with him throughout the level, most players would want to see Jeff's threat eliminated. Originally, this was achieved by ending the chapter with Jeff getting dragged off by antlions, demonstrating to the players that antlions were a much more dangerous threat before having the player fight them themselves. However, this left some playtesters feeling empty in that they personally could not get revenge on Jeff for causing them so much stress. Other problems emerged: some playtesters ended up feeling sorry for Jeff and did not necessarily want to see him killed, and the antlion attack came off as random and meant Jeff presumably died offscreen, preventing a sense of closure for the chapter before the players dealt with the antlions. The trash compactor served as the solution: it gave players the chance to decide Jeff's fate and leave with a sense of closure, whether by killing him or just leaving him trapped (though most players tend to choose the former).
The constant experimentation and testing paid off: on Half-Life: Alyx's release, Chapter 7 was singled out as one of the highlights of the entire game, with critics noting its unique take on the survival horror genre in a VR game.
Earned by staying near Jeff for 10 seconds.
Earned by killing the Headcrab that keeps alerting Jeff.
Hold Your Liquor
Earned by holding a rolling bottle before it alerts Jeff.