The Lost Soul in Doom 3 is a head-sized flying Cybrid demon consisting of a human female face mounted on a flying rocket engine contraption. Its jaws are torn apart to allow it to open its mouth wider than a normal human could and its teeth have mutated to long, pointy, needle-like teeth. When alerted, they make a high-pitched shriek, accompanied by the sound of the rocket engine that powers them.
They hover through the air at average speed and attack by abruptly rocketing in a straight line towards the player at high speed and ramming into them, similar to the classic Lost Souls. Unlike the classic Lost Souls, Doom 3 Lost Souls can cover a larger area and can change course if necessary. The classic Lost Souls are giant, flaming, flying skulls with sharp teeth, whereas the Doom 3 Lost Souls are flying, flaming human heads. The Lost Soul is usually found in packs with other Lost Souls (which was often the case in the classic Doom games as well). The Lost Souls can be problematic opponents if the player does not manage to kill them quickly. They first appear in the Mars City Underground in a more skull-like form (similar to its classic version), and they appear to be responsible for possessing humans and turning them into Zombies - the first zombie encountered being Jonathan Ishii. They also appear to stalk the player throughout the Underground, but they don't attack. Their first appearance as enemies is in the EnPro facility.
The first proper appearance of a Lost Soul as an enemy occurs in a grotesque cutscene with Theresa Chasar, one of the few female UAC employees. When the marine approaches, she whispers "help me", then her facial skin starts to shrivel inwards while acquiring a horrible pale color, and a Lost Soul bursts out of her head, ripping out her spinal cord in the process. It could be possible that a Lost Soul needs to manifest itself within a body; this is supported by the cutscene and the rest of the Lost Souls could have come from the bodies with the damaged faces as well as the ones without heads. However, this might just have been a coincidence (teleporting at the right time) but does not explain Theresa's behavior, heavily supporting the first theory.
They are also considered some of the most annoying enemies, as they can often knock the player off a ledge to their death.
While the Doom 3's version of the Lost Soul is weak compared to its classic version (a single punch or 2-4 pistol bullets can take them down instantly), it is a much more difficult target, due to Doom 3 being fully 3-D rather than pseudo-2-D, which made the old version easier to hit. This, plus the lack of space for the player to dodge their charge attacks, means they should not be underestimated, as one missed shot can give a Lost Soul enough time to retaliate against the player, which makes them more of an annoyance than the classic version, especially when attacking in swarms. The Machine Gun or the Plasma gun are the most efficient weapons for taking down a horde of them, thanks to their high accuracy and rapid rate of fire.
One risky way to kill them is to use the Shotgun. Fire while it charges at you and you should get an insta-kill if it's about 2 feet away from you. Make sure no one is flanking you before you try this.
The Resurrection of Evil and The Lost Mission expansion packs replaced the Lost Souls with a new monster called the Forgotten One, which bears a closer resemblance to the classic Lost Soul (hence the name "Forgotten One", cause they had been forgotten since the classic Doom games). Just use the Grabber on them.