A human-vampire hybrid is the child that results in the successful mating between a human and a vampire, sharing many of the traits of both. A hybrid can only be conceived by a female human and a male vampire. Female vampires cannot reproduce; this is because a female's body has to change to bear the child, and since vampires are frozen at the state of transformation, pregnancy is impossible. So far the only known hybrids are Renesmee Cullen, Nahuel, and Nahuel's three half-sisters.
Physical appearance
Human-vampire hybrids have some physical traits of 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. However, unlike the pale skin of vampires, their skin coloration is more like humans, with their skin tones varying from the father (Joham's rich dark brown skin to Nahuel and most likely his sisters) and (Edward's pale white skin to Renesmee) along with inheriting their father's facial features, which is how Renesmee face appears most like. and with color in their cheeks due to the presence of flowing blood beneath the surface. Their eyes are not the crimson or gold of vampires either, but the colors of their human mother's, such as chocolate brown (Bella to Renesmee) or warm teak (Pire to Nahuel). Unlike the vampires who have deep purplish-bruise looking eyelids and are a constant color all over, Hybrids have pale lavender colored eyelids, pink lips, and pink palms on their hands and probably feet too. Their hair color is most likely, inherited from their father, (Edward's and Renesmee's bronze) while the overall texture of their hair could be inherited from the maternal side of the family ( i.e. How Renesmee has her grandfather, Charlie's, curls.) In sunlight, vampire-human hybrids do not glitter as vampires do, but do appear slightly luminous. These human-like qualities make it easier for them to blend in with humans, even during bright days.
Due to their beauty and super-human strength and speed, from a distance they may be mistaken as Immortal Children.
Biology
Human-vampire hybrids share some biology between humans and vampires. They possess supernatural strength, speed, and dexterity close to that of vampires, but falling slightly short. Their skin seems to be as impenetrable as the skin of other vampires, though it is not clear if they have the same ability to 'repair' themselves after an injury. Their heightened senses seem to be as perceptive as normal vampires, since Bella comments that the visions Renesmee show her appear as if they were through her own eyes and ears. They appear to have perfect memory recall, like vampires, and mentally develop at a rate far faster than both their true age and the age they appear, speaking in complete sentences and walking just days after birth. They also can possess supernatural gifts, possibly influenced by the gifts of their parents (as in Renesmee's case). So far no known hybrid females have venom, while Nahuel, the only known hybrid male, does; it is unclear if this is due to gender or random chance.
There is however a recurring theory about hybrid vampires. A male hybrid has venom in their system because they inherit more genes from the vampire father than the human mother. Females do have venom albeit but not as potent or infectious ( this theory is supported by the demonstration vampire venom has on its prey; Bella barely needed to hold down the lion she was feeding from, because the venom was already burning its system.) therefore Renesmee and Nahuel's sisters do have venom but it's potency is only capable of incapacitating an animal or a human being.
Unlike vampires, human-vampire hybrids have a functioning heart with blood flowing through their veins, must breath to survive, and sleep as humans do. They are capable of eating and gaining nourishment from both normal food and blood, though it appears blood, particularly human blood, is their preferred diet. Their body temperature is unusually warm, even compared to humans, just a few degrees below the norm for shape-shifters.
Also unlike vampires, human-vampire hybrids age during their first few years, developing from infant to full maturity approximately seven years after their birth, at which time they physically appear about seventeen years old. Once they reach full maturity, they stop aging completely and become immortal, as demonstrated by Nahuel's lengthy life of over 150 years with no apparent change.
It is currently unknown whether hybrids can reproduce. The ability seems likely, since the primary limitation for female vampire pregnancy is the fact they do not change, which is not the case for early human-vampire hybrid development. Even at full maturity when hybrids do not appear to change, hybrids are able to digest and use normal food to sustain their bodies, which may indicate their body is still capable of metabolism and change but simply doesn't age.
Like male vampires, male hybrids have sperm - an indication that reproduction is possible.
They have 24 chromosome pairs, the same as shape-shifters, which leads to speculation as to whether Renesmee can reproduce with Jacob.
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 first and only book in the Twilight saga where hybrids make an appearance. When Bella and Edward Cullen went on their honeymoon, Bella fell pregnant. The pregnancy seemed fast-forwarded; Bella and Edward had only had intercourse for the first time 17 days prior 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 the child out of fear for Bella's safety. Rosalie, who has always wanted a child, agrees and guards Bella during the pregnancy, preventing an 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, preventing 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 baby's condition. The blood worked to improve her health, but the stronger Bella got, the stronger the hybrid got, too. By now it was involuntarily breaking her bones and bruising her body due to it's abnormal strength. After further research, the Cullens discover that the hybrids usually rip their way through the vampire-like amniotic sac and out of the mother's body, killing the mother in the process. They decide emergency vampirization for Bella is the only likely way to save her after birth, assuming she survives 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 tow her away while Edward works furiously over Bella. The baby was born, but Bella was slipping under fast, even having her heart stop several times. Jacob refuses to hold the child and an in-control Rosalie comes back to take her, Renesmee, downstairs.
Three days later, Bella wakes up from the transformation, and sees Renesmee after hunting. Renesmee has grown at an incredible pace. She does not look like a three days old baby at all. Renesmee is very intelligent and still grows at a speedy rate. She prefers blood to human food, though she could survive on both.
From far away, Renesmee would look like an Immortal Child. She has a faint blush in her cheeks because there is blood pumping around her body and she also has a heartbeat, which sounds like a humming bird's wings fluttering.
Later, we are introduced to Nahuel, another hybrid, who shares that he stopped growing approximately seven years after his birth, when he reached fully maturity. He shares many of the same physical traits as Renesmee, but 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 legend about human-vampire hybrids are about the dhampyres. 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.