Hybrid

A human-vampire hybrid is the result in the successful mating between a human and a vampire, sharing many traits of both. A hybrid can only be conceived by a female human being and a male vampire. Female vampires cannot reproduce; due to the inability of vampires' bodies to undergo changes after their transformation. So far the only known hybrids are Renesmee Cullen, Nahuel, and Nahuel's three half-sisters (all of which remain unnamed in the series).

Physical appearance
Human-vampire hybrids share some physical traits with both humans and vampires. They are described as incredibly beautiful throughout their development from infant to adult. Like vampires, they have well-developed physiques and smooth, flawless skin. In sunlight, human-vampire hybrids do not glitter as vampires do, but do appear slightly luminous which allow them to more easily blend with human beings, even during bright days.

Due to their beauty and superhuman abilities, from a distance they may be mistaken for immortal children.

Biology
Human-vampire hybrids share some biology with humans and vampires. They possess supernatural strength and speed. Their dexterity is barely less than that 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. Though both male and female hybrids possibly show the supernatural healing capacities of vampires, only male hybrids may perhaps 'repair' themselves after such an injury as dismemberment as completely as vampires, since females lack the venom necessary for the process. 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 both 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 venom, though Nahuel does; it is unclear if this is due to gender or chance.

There is however a recurring idea about hybrid vampires and the venom they produce. Female hybrids, such as Renesmee and Nahuel's three sisters, most likely 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. A hybrid that is venomous like vampires may also have the same venom based-fluids throughout their body that full vampires have.

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 it appears blood, particularly human blood, is their preferred food. Their body temperature is unusually warm, just a few degrees below that of shape-shifters.

Also unlike vampires, human-vampire hybrids age early in their life, reaching full maturity approximately seven years after their birth, when they physically appear about seventeen years old and stop aging completely, as demonstrated by Nahuel's lengthy life of over 150 years with no further apparent change.

It is currently unknown whether hybrids can reproduce. It may be possible they can, since the primary limitation for female vampire is their inability to change - not the case for human-vampire hybrids. Even at full maturity, hybrids are able to digest normal food, which indicates an active metabolism, aging being the only aspect affected. Like male vampires, Nahuel has sperm.

Like shape-shifters, they have 24 chromosome pairs, but it is unknown whether this is due to a sampling error or chromosome rearrangement. There is, however, speculation that Renesmee and Jacob may reproduce.

The hybrids are a blind point in Alice's vision, another similarity with the shape-shifters. 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
Breaking Dawn is the only book in the Twilight saga where hybrids make an appearance. Bella became pregnant on her and Edward's honeymoon. The pregnancy seemed very rapid; Bella and Edward had only had intercourse 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 Hale 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, whom 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 comment 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 her health, but the stronger Bella got, so did her daughter. By now, the child was involuntarily breaking her bones and bruising her body, with her abnormal strength. 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 emergency vampirization for 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 worked furiously over Bella. The baby was born, but Bella was slipping under fast, 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 woke up from the transformation and saw 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 a distance, 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.

Later, Nahuel, another hybrid, was introduced, with the information that he stopped growing at approximately seven, when he reached fully maturity and that he shares many of the same physical traits as Renesmee. However, unlike Renesmee and Nahuel's three hybrid half-sisters, he is venomous and therefore capable of creating an immortal (which he did as a newborn to transform his aunt Huilen). Nahuel tells of his father, Joham, who thinks he is creating a super race.

Outside the Twilight series
The most famous legends about human-vampire hybrids are about the dhampyrs. Dhampir (also dhampyre, dhamphir or dhampyr) in Balkan folklore is the child of a vampire father and a human mother. Dhampirs often have powers similar to a vampire, but none of their weaknesses (though the reverse can occur, as well). A dhampir is believed to be unusually adept at killing and detecting vampires.

Other legends are those about Cambions, which are the closest to the Twilight Saga. As they go. Cambions are the children of an incubus and a woman or a succubus and a man, and they are extremely beautiful and often manage to get humans to do their bidding, very close to what Renesmee is.