The Hunter Mutation:

The Hunter is the least visibly mutated Special Infected and superficially resembles a Common Infected. It has yet to be explained how the Hunter's mutations onset, whether it be genetic, biochemical, or a Hunter-specific strain of the virus, but the mutations seem to be sex-linked as only male Hunters have been observed. The Hunter's reaction to the Infection has granted him an increase in lower-body strength.

While not creating massive muscle growth like the Tank, this added strength has given the Hunter the ability to repeatedly jump and pounce from long distances and rapidly scale the sides of buildings with ease, as well as toughened his legs in order to withstand the great distances and heights he can achieve. Decreased pain response visible in all sufferers of the Infection also results in the Hunter not suffering from perpetual lactic acid build up from muscle overexertion or impact damage after experiencing a large fall.

At the same time, there seems to be some sort of muscle delay after an initial attack (post-attack recharge) where the Hunter is not able to move as swiftly. One of the most notable physical observations is that the Hunter lacks eyes. Whether this is self-inflicted, a by-product of the Infection, or both, is unknown.

The extent of damage to the Hunter's sight suggests his other senses are far more developed than those of typical Infected, allowing him to hunt with far greater efficiency. It is possible that the screeching noises he makes and the growling are his own echolocation method. He might also use scent to tell the Survivors apart from Infected. As with the other Special Infected, he is not distracted by such high-frequency noises as Pipe bombs or car alarms going off. Hunters also appear to be the only Infected with a self-defense reflex: when shot at a distance and knowing they cannot pounce, the Hunter will sometimes jump away into safety. 

Riley Essyl:
Riley is an average Hunter with no real memories of his life before the infection (as is the case with any infected). His fine motor functions aren't always all there, as he struggles with things such as opening doors, etc. Unlike most infected, Riley can understand some bits of the English language and he's learned how to mimic some small words in order to communicate back--or even to lure survivors into a trap via vocal mimicry. Though, on both topics, if there's something he can't do or can't communicate, he seems to get easily frustrated and goes on a mini tantrum. Though it's uncertain where in the biochemical mutation process it stems from, Riley has shown to experience ruts and will become increasingly territorial during. His eyes are not nearly as heavily damaged as most Hunters are and, while his other senses are still as sharp as any other Hunter, he's still retained some of his vision, though weak. He is sensitive to the light and therefore more active at night and pipe bombs (and general loud noises) have the opposite effect on him. Because of losing his pack, he can be heard whimpering and crying at night sometimes.

When he was alive and well, Riley was more of a quiet person who kept to his small group of friends and his studies. He had a heavy interest in virology even before the apocalypse started and was in school, working towards his doctorate. He's always had a bit of an attachment problem (primarily over attaching) and when the world started to go to shit, it just exacerbated that problem. He clung to fellow survivors as a lifeline. When he was sent to scout for supplies one day, he was attacked by a Hunter. The Hunter bit into his left forearm as Riley tried to defend himself. He managed to kill the Hunter before it did any lasting or substantial damage. But while he was immune to common strains, he was not immune to the Hunter strain. This caused his eventual turning and downfall.