Hybrid

"Half mortal, half immortal. Conceived so, and carried by this newborn while she was still human."

- Aro's comment on Renesmee Cullen

A human-vampire hybrid, also known as half-vampire, vampire hybrid, half-human and half-vampire, and half-immortal, is the resulting offspring of a successful mating between a human and a vampire, and shares many traits of both species. Male vampires have the capacity to pass on genetic material with a human female partner. However, it is beyond the ability of most vampires to be so close to a human physically and still resist the lure of his/her blood. Because of this, vampire hybrids are an extremely rare species.

There are currently five known hybrids: Renesmee Cullen, Nahuel, and Nahuel's three half-sisters, Serena, Jennifer, and Maysun.

Conception
The whole process is extremely risky and can easily fail due to the fragile human body.

A hybrid is conceived by the sexual intercourse between a female human and a male vampire. Because most vampires have a hard time resisting the urge to kill humans, it takes a high level of self-control to be so close to the human and still resist the temptation of her blood.

Once the woman is impregnated, the fetus inside her grows much faster than human fetuses, and will grow extremely strong overtime. Should it cause too much trauma to her before birth, it can easily destroy its own existence by killing the mother. If the fetus is not often fed with blood, it will involuntarily deteriorate its mother's health, making the chances of survival even smaller.

The fetus is grown to full term one month after conception, and will utilize its own strength and teeth to break its way out. This will, however, result in the death of the mother, as the damage is too much for her body to take. The only known way to save the mother after the fetus's birth is by injecting vampire venom into her body and change her into a vampire. So far, only Bella Swan has survived this process.

Physiology
Vampire hybrids have both human and vampire traits.

They are immensely beautiful. Due to their beauty and superhuman abilities, they can be mistaken for immortal children from a distance when they are still maturing. They have well-developed physiques and smooth, flawless skin, which is paler than a human's but not as pale as a vampire's.

Unlike vampires, they have a functioning heart with blood flowing through their system. A hybrid's beats faster than a human's. They can sleep like humans and are capable of eating and gaining nourishment from both human food and blood, though they prefer blood, particularly human. Their body temperature is unusually warm, just a few degrees below that of shape-shifters.

While vampires' skin sparkles like diamonds when exposed to sunlight, hybrids' skin shows a weak illumination, a feature that allows them to blend in more easily with human beings even during bright days.

Hybrids do possess the mixed scent of vampires and humans. This gives them a unique scent all their own, balanced between being both appetizing as a human's and sweet as a vampire's. Bella Swan described this as just enough vampire scent to keep the human scent from being too overwhelming.

They have 24 chromosome pairs, but it is unknown whether this is due to a sampling error or chromosome rearrangement. It seems, however, unlikely that they have any relation to the shape-shifters' chromosomes.

As with shape-shifters, hybrids are a blind spot in Alice's vision. This is later explained to be due to the fact that Alice has never experienced life as a shape-shifter or a hybrid, and therefore cannot relate to it.

It is currently unknown whether hybrids can reproduce or not. It may be possible that they are able to, since the primary limitation for female vampires is their inability to change - not the case with hybrids. There has been speculation that Renesmee and Jacob may reproduce, but there is not enough information to confirm this. Even at full maturity, hybrids are able to digest normal food, an additional sign of an active metabolism. Like male vampires, Nahuel produces sperm, though eventual children have not been mentioned.

Growth rate
Human-vampire hybrids grow rapidly early in their life, reaching full physical maturity at approximately age seven, when they physically appear in their late teens and will cease aging altogether, allowing for the same unchanging state as vampires, as demonstrated by Nahuel's lengthy life of 150 years with no further apparent change. Nahuel's oldest sister, Serena, is known to have lived for over 200 years.

Powers and abilities
They are supernaturally much stronger and faster than humans, with sharper senses, but are not as strong, fast, or sharp as a pure vampire (Bella described Nahuel as "not quite as fast nor fluid in his run as the vampires with him"). Like the hybrid's amniotic sac, her/his skin is almost as durable as a vampire's. They have perfect memory recall and develop mentally at a rate far faster than their chronological and biological age, speaking in complete sentences and walking just days after birth.

So far, no known hybrid females have been shown to be venomous, though Nahuel, the only male hybrid, has; it is unknown if this is generalizable. Despite both male and female hybrids showing the supernatural healing capacities of vampires, in such a case as an injury like dismemberment, only male hybrids may perhaps 'repair' themselves as completely as vampires. Since the known females apparently lack the venom necessary for the process, it may be supposed that their healing is not as extensive as males, and they would not survive such an injury. There is, however, a recurring idea about hybrids and the venom they produce. Female hybrids, such as Renesmee and Nahuel's three sisters, may have received prominent traits from their human mother, hence their lack of venom. Nahuel seems to have received most traits from his vampire father, which would make him venomous. It would appear that a venomous hybrid has the same venom-based fluids throughout their body as a full vampire. Non-venomous hybrids may have them to a lesser degree (as they have the sweet-smelling scent of vampires). Nahuel, a venomous hybrid, may display the same scent that full vampires have, without even a hint of human blood (his recently turned aunt Huilen was not tempted by the scent of his blood).

Supernatural abilities
According to The Twilight Saga: The Official Illustrated Guide, hybrids can develop supernatural capabilities like certain full vampires, though not all do. Renesmee is the only hybrid known to show supernatural gifts, which are inherited from her parents, and altered to work differently. It is unknown if other hybrids also show them, or if they are inherited or uniquely their own.

Breaking Dawn
Hybrids only appear in Breaking Dawn. Bella became pregnant on her and Edward's honeymoon. The pregnancy progressed very rapidly; Bella and Edward had only made love for the first time 17 days before her conception and her period was only 5 days late, yet there was a small but noticeable bump around her hip. Bella decided to keep the child, as the hybrids do have a lovable presence for the mothers. She asked Rosalie for help when Edward made it known that he wished to abort it out of fear for Bella's safety. Rosalie, who always wanted a child, agreed and stood guard over Bella during the pregnancy, preventing the abortion.

However, Bella's health was rapidly deteriorating due the strength of the hybrid, which she believed was a boy and tentatively named "EJ" (standing for Edward Jacob). The pregnancy also deprived Bella of her appetite, stopping her from eating normal food. From a snide idea that Jacob was thinking, Edward realized that the appetite of the baby may be closer to vampires than to humans. Carlisle and Rosalie agreed that Bella should try drinking human blood to improve her and the child's condition. The blood did improve the health of the fetus, and Bella's health improved as a result of that. By now, the child was involuntarily breaking her bones and bruising her body.

After further research, the Cullens discovered that the hybrids usually rip their way through the vampire-like amniotic sac and out of the mother's body, killing her in the process. They decided that turning Bella was the only likely way to save her after birth, assuming she survived the delivery process with a beating heart. When Bella accidentally spilled a cup of blood on the couch, she reached for it too quickly, causing a detached placenta and triggering the birth. Rosalie, Jacob and Edward were working on getting the baby out, when Rosalie lost her concentration and gave way to her thirst. Jacob and Alice led her away while Edward started working furiously over Bella. The baby was born, but Bella was on the brink of death, even having her heart stop several times. Jacob refused to hold the child, now called Renesmee, and an in-control Rosalie came back to take her downstairs.

Two days later, Bella's transformation was complete and she was allowed to see Renesmee after hunting. Renesmee had grown at an incredible pace, no longer looking like a three days old baby at all. Renesmee was very intelligent and still grew at a fast rate, showing a preference for blood instead of human food.

From afar, Renesmee would look like an immortal child. She had a faint blush in her cheeks because her heart was pumping blood through her body - her heartbeat sounding like a hummingbird's wings fluttering.

Nahuel was introduced later, with the information that he stopped growing at approximately seven, when he reached full maturity, and that he shares many of the same physical traits as Renesmee. However, unlike Renesmee and his three hybrid half-sisters, he is capable of creating an immortal (which he did, just after birth, when he bit Huilen). Nahuel tells the Volturi of his father, Joham, who thinks he is creating a super race. While he speaks with distaste about his father, he pleads with Aro not to harm his sisters.

Outside of Twilight
Male vampires are known to father children with human women - said children are known in the Balkan folklore as dhampirs. They're reported to show many of the vampire powers, but not their weaknesses, and are known to be vampire hunters.

Another creature that bears mentioning is a demon who is said both to induce nightmares in sleepers and engage in sexual activity with them, assuming the name "succubus" when it targets men and "incubus" when it targets women. Lilith is the most notable example of them. Occasionally, offspring are born from this intercourse, Merlin being the best known example. However, since the demon is sterile in itself, it must collect semen from the men it has sex with for later use with the women it seduces. These kinds of legends are found in various forms in European, Middle Eastern and South American folklore.

In Marvel Comics, the vampire hunter Blade is also a dhampir of sorts. His mother was changed on the night he was born. As a result Blade was born with all the powers of the vampires from his universe (as well as the need for blood), but not their weaknesses (earning him the nickname Daywalker).