Hybrid

A human/vampire hybrid is the result of a successful mating between a human and a vampire, and shares many traits of both. A hybrid can only be conceived by a human female and a vampire male, as female vampires' bodies are unable to change to accomodate the growing foetus while a male vampires' sperm count is comparatively unaffected by the change. So far, the only known hybrids are Renesmee Cullen, Nahuel, and Nahuel's three half-sisters, who remain unnamed.

Physical appearance
They are described as incredibly beautiful throughout their whole growth from infant to adulthood. Due to their rapid growth, hybrids reach full maturity at the age of seven, with the looks of someone of the age of seventeen. As with vampires, they have well-developed physiques and smooth, flawless skin, which is paler than a human's but not as pale as a vampire's. In sunlight, human/vampire hybrids do not glitter as vampires do, but appear slightly luminous, a feature that allows them to blend more easily with human beings even during bright days. Due to their beauty and superhuman abilities, they may be mistaken for immortal children from afar.

Biology
They possess supernatural strength, speed and dexterity just short of those of vampires (Bella described Nahuel as "not quite as fast nor fluid in his run as the vampires with him"), and their skin seems to be as impenetrable as that of vampires. Their senses are heightened to vampire level, since Bella comments that the visions Renesmee shows her appear as if they were through her own eyes and ears. They appear to 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. They can also possess supernatural gifts, possibly influenced by those of their parents (as in Renesmee's case).

So far, no known hybrid females have been shown to be venomous, though Nahuel, male, does; it is unclear if this is due to gender or chance. Despite both male and female hybrids showing the possibility of 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 lack the venom necessary for the process, it may be supposed that their healing is not as extensive as male, and 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 of 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). Non-venomous hybrids, such as Renesmee, do possess the sweet scent but to a lesser extent (Renesmee's scent can be described as being as tempting as a human's). This gives them a unique scent all on their own, balanced between being both appealing (as a human's) and sweet (as a vampire's).

Unlike vampires, human/vampire hybrids have a functioning heart with blood flowing through their system. They can sleep like human beings. They 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.

Human-vampire hybrids grow rapidly early in their life, reaching full maturity at approximately seven, when they physically appear about seventeen years old and stop aging, as demonstrated by Nahuel's lengthy life of over 150 years with no further apparent change.

It is currently unknown whether hybrids can reproduce or not. It may be possible they are able to, since the primary limitation for female vampire is their inability to change - not the case with hybrids. 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.

They have 24 chromosome pairs, but it is unknown whether this is due to a sampling error or chromosome rearrangement. It seems, however, unlikely they have any relation to the shape-shifters' chromosomes. There has been speculation that Renesmee and Jacob may reproduce, but there is not enough information to decide one way or the other.

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

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 birth 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 of 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 from 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 suddenly, 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, by now called Renesmee, and an in-control Rosalie came back to take her downstairs.

Three 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 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 dhampyrs. 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 is 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, the latter perhaps explaining the depiction of Joham and Nahuel in the Twilight series.