Although bearing the greatest resemblance to William's former self, his upper torso is far more muscular due to a sudden increase of muscle mass. Due to the development of the G-virus, damaged tissue is replaced with a more heavily-muscled version. This characteristic of the virus led to a disproportionate right arm, and a large tumor-like eyeball has developed within the shoulder. Also, two vestigial arms can be seen beginning to grow in the torso, foreshadowing his later mutations. It is implied in cutscenes, and by his initial actions, such as directly pursuing and attacking the U.S.S. team that had assaulted him, that Birkin is, at least initially, still in control of himself in this form.
Conversely, when Annette recollects William Birkin's transformation and his slaughter of the Umbrella Security Service, G is shown slashing the soldiers with talons. When G first appears, though, no such claws are visible whatsoever; thus, one can assume that the talons were either lost in a process of devolution (contrary to the G-virus' properties), or, more likely atrophied away following lack of use in any further combat (a process much more akin to the expected course of action of the G-virus, due to its unpredictable abilities and volatile mutations). It should be noted, however, that G's arm mutates further after tearing off the guardrail, bulging grotesquely and gaining the spike of bone surmounting the eyeball in the shoulder. It should be noted that G first form is the only one of his humanoid forms that have shoes.
Birkin mutated into this form to fight with Claire Redfield, on her way down to his underground facility. During this mutation, his pants are torn apart to the mid-thigh and the remains of Birkin's body were replaced with new, stronger parts: The head was pushed down into the thorax, the digits of the right hand sprout into massive talons comprised of bone, two tiny vestigial limbs also developed lateral to the abdomen underneath the original arms, and a new rudimentary skull grew to take the place of the original. Also, a circular growth of spikes has begun to grow in his chest, yet again foreshadowing his next transformation.
The creature already has nearly complete lower-arms, and Birkin's original head has all but vanished. The claws are much longer, and there is an eye on G's back. Its attacks are different now: it has a powerful stab preceded by a noticeable wind-up (exposing the eye on its back in the process), a jumping slash, a quicker stab, and its old double swipe from the original game. Its only real weak point is the exposed eyes on various points on its body.
Interestingly, in Darkside Chronicles, this form of G seemed to recognize Annette to some extent. It stopped and examined her for a few moments before she drew her gun on it, which it seemed to recognize as a threat. It then killed her by stabbing her in the stomach.
Perhaps the least grotesque of the various forms that William Birkin assumed following the injection of the G-virus into his body. The host by now has shed all of its integument-like skin, making it nude and revealing its musculature, whilst hundreds of spike-like projections rise prominently into an apex from a point in its upper left thorax; the visage of William Birkin, on the other hand, slowly begins to merge and fade away into the abdomen. This mutation is also William's final humanoid form.
In addition to its right hand, the beast's left hand also possesses deadly talons now; in fact, the abomination has grown in height to nearly eight feet; thus, its primary arms have also elongated beyond normal human proportions. They have retained the claws, but the right hand has talons formed from bone, whereas the left hand has claws formed from nails. The vestigial arms of the prior stage, meanwhile, have also matured into functional limbs with talons, and the rudimentary head, aside from having completed its new, harder bone skull, now sports a lipless grin, bearing its razor sharp teeth, that makes it vaguely resemble a skinless form of the facial features of the Nemesis-T Type. Finally, a secondary, smaller eyeball-like tumor has developed in the left thigh of the creature. This form is the one with the most bilateral symmetry, and to some, it resembles a demon in appearance.
Furthermore, when G is encountered after crashing through the ceiling shortly before its complete transformation into its hexapodal fourth form, the appearance of its third form has changed slightly. The spikes located on its chest have begun to open and pulsate slightly, hinting at the appearance of its fourth form. It's head has also changed into the design of its fourth and fifth forms.
The mutation in this stage involves G mutating from a standing equilibrium as a bipedal to a hexapedal base on the ground involving the utilization of its four primary limbs to maintain movement. The skull and maxilla of the creature have elongated to accommodate its massive mandible, a gaping, circular cavity surrounded by rows of spike-like projections, which has now fused into the many spikes that formerly covered the thorax of the beast. Despite its appearance, however, this stage of G is extremely agile, able to sprint and leap all about a room with ease, and the aforementioned spikes around the mouth are more than capable of impaling its prey with extreme ease, given the superior agility and speed of the stage. However, in order to maintain its speed, it needs to constantly eat more. The 2 eyes formed on its shoulders could either be rudimentary, or be the actual eyes, and make the 'eyes' on its 'head' could simply be for show, like on a butterfly or moth.
This stage moves around with an odd gait, often compared to a dog or wolf. It jumps about from pillar-top to pillar-top, and often jumps down in front of a character. It will either swipe at them with a small forearm, or more dangerously, it will grab them and gnaw them with its massive fangs.
By far the most grotesque of all the forms that G mutated into. After extreme physical trauma to G, it has become a shapeless blob of flesh, spikes, and tentacles, slowly dragging itself via attachment of its largest tentacles to a stationary object and subsequent use of its lesser tentacles to slime forth. This final form was killed only after the train which it had invaded was consumed by a massive explosion following the automatic activation of its self-destruct sequence, as the damage it had sustained at cellular level was too high for even the G-virus to counteract.
Since the G's infected William Birkin was killed at this point, it is unknown if it would have continued to mutate into further forms, it is likely that it would not, due to being a mere shapeless blob, there would be nothing left to mutate into. Yet in the Wesker Report II, Wesker states that the G-Virus will continue mutating the host until he or she dies. It should be noted however that the G-Virus did not produce this mutation because of evolution, but because it was heavily damaged and was a response to that. Most notably, even at this stage, the G has at least some rudimentary form of intelligence, as it recognized that the train it was on was about to self-destruct when the timer expired. Fortunately, the train detonated, leaving no time for the monster to escape.