Werekin

Were-Creatures, most notably werewolves, are creatures that are not only one species but two at the same time. Some werewolves are either allies with vampires or against them. Another type of Were-Creatures are Werecats. In Hellsing the only two were-creatures are The Captain and Schrodinger from Millennium.

Werewolves
A supernatural creature that transforms from a human into a humanoid wolf-monster under a full moon. Like vampires, werewolves are vulnerable to silver or silver colored items. When the sun rises, they revert into their human form with no memory of what happened.

The Captain has the ability to become a werewolf at will, shifting between human, wolfman, and full wolf forms, and possessing unprecedented power even while in human form.

Abilities
Super Strength

Werewolves are several times stronger than average humans, able to lift large objects that normal humans could not and inflict incredible damage with physical attacks.

Immortality

The Captain appears to possess immortality as a werewolf, having not appreciably aged during the fifty years of Millenium's operations. It is likely that he could only perish if killed with silver.

 Super Durability 

The Captain's damage resistance was extremely high. He could jump from a high-flying zeppelin to the ground below with ease, resist Walter's microfiliament wires, and shrug off massive amounts of injury from anything besides a silver weapon.

Shapeshifting

The Captain can transform from a human to an enormous silver wolf and back. He is also able to shift into a hybrid "wolfman" form or turn into mist just like a vampire. It is unclear if this voluntary shapeshifting is a sign of the Captain's great power and skill, an artificial enhancement from the Doctor, or simply a natural trait of all werewolves in the Hellsing universe.

 Heightened Senses, Speed, and Agility 

The Captain has incomparable speed and agility, allowing him to cover great distances in moments and dodge most attacks. He also possesses keen battle sense.

Weaknesses
Silver: Just like vampires, a werewolf is weak against silver weapons. The most effective method is a silver bullet, but anything silver, including a silver tooth, proves to be potent.

Possible loss of speech: The Captain is entirely mute. It is not clear if this is a side effect of being a werewolf or simply because he refuses to talk.

Notable Werewolves
The Captain

Werecats


Although not as popular as werewolves, werecats were said to have many of the same abilities as a werewolf, but with cat-like traits instead of a wolf. In the Hellsing universe, werecats appear as humans with furry cat ears. Millenium's Warrant Officer, Schrodinger, is the only known werecat/catboy in the series.

Abilities
Omnipresence

Schrodinger is able to exist wherever, however and whenever he chooses. This allows him to bypass all known security systems, travel great distances with a thought, appear in multiple locations at once, and even manifest in Seras Victoria's subconscious mind to speak with Zorin Blitz.

 Nigh Invulnerability/Immortality 

Schrodinger is able to recover from seemingly any injury, no matter how severe. He had most of his head destroyed at least twice in the series, only to appear perfectly fine in later scenes. Although it is never fully explained in the series, this ability is implied to be an extension of his omnipresence powers; if wounded or killed, Schrodinger simply visualizes himself to be somewhere else, alive and unharmed.

Weaknesses
Perception of Self

According to the Major, Schrodinger's ability to be "everywhere and nowhere" requires him to be able to perceive himself as an individual. If he isn't able to recognize himself, he is "nowhere", and his body fades away completely, becoming "a cluster of imaginary numbers". This weakness becomes the crux of the Major's plan to destroy Alucard; by tricking the vampire into absorbing Schrodinger (and thus gaining his abilities), the Major hoped to erase Alucard from existence by leaving him unable to identify himself among the millions of souls that made up his being.

Notable Werecats
Schrodinger