Strigoi are magically reanimated human corpses which are inhabited by the spirits of the deceased person and who closely resemble the living human they were before their transformation. 

Strigoi feed and survive on the blood of the living, typically on that of humans, and they can also transform other humans into more of their kind by sharing their own blood with them.

Transitioning

In order to complete the transition into a Strigoi, the person must then consume a significant amount of human blood, at least a sip's worth, within the next twenty-four hours, or else they will die. Strigoi can only feed on human blood in order to complete their transformation—animal blood or Strigoi blood is not sufficient to finish the transformation from human to Strigoi. However, in this case, "human blood" can include the blood of a human witch.

A Strigoi in transition is neither truly living nor truly dead until they make their choice to either complete their transition or to abstain from feeding and ultimately die. This puts the semi-Strigoi in a strange liminal space between the two states that can affect them in different ways.

Sireline

Every Strigoi who has ever existed is descended from one of their kind known as the Alpha Strigoi, who, unlike the majority of Strigoi, was turned not by dying with Strigoi blood in his system, but who was instead transformed using a spell that was cast using Dark Magic at some point in history.

Physiology

"Look. We're Strigoi. We're a toxic species. Whether you wanna blame it on magic or biology, it's just who we are. When a human gets ravenously hungry, they eat a double cheeseburger. We kill people. Some of us are more extreme than others, but that's just who we are. When humans want to break free of complicated relationships, they go to therapy. Strigoi get a pass."

Blood is the main component of a Strigoi's diet, be it human, animal or Strigoi blood.

Blood (especially human blood) does not only satisfy a Strigoi's thirst, but it also heightens their supernatural abilities, making them stronger, faster, and able to heal much more quickly than they would after not eating for a significant period of time. The longer a Strigoi abstains from blood, the more likely the Strigoi will become feral until they are eventually unable to resist feeding on whomever they come upon. In many cases, a Strigoi who has been starved for a long period and is risking desiccation will eventually become so overwhelmed with a hunger that they lose all sense of their humanity and rationality altogether and finally give in to their bloodlust. It has been demonstrated that Strigoi blood, just as with human and animal blood, can be used to revive a Strigoi who has been desiccated (even if they ordinarily do not partake in the consumption of Strigoi blood), and it can also help a badly injured Strigoi heal more quickly if human or animal blood is not available. However, human blood is most compatible with a Strigoi's dietary needs, and because of this, the majority of Strigoi rely on it as their sole food source. Despite their main food source being referred to as "human blood," this term also extends to the blood of both witches and werewolves who are in their human form, as their blood seems to be functionally identical to that of regular humans.

Due to their acute senses, a Strigoi can detect whether or not a human has recently consumed alcohol or drugs, because their blood tastes unclean and can leave a Strigoi with a chemical aftertaste. This also extends to vervain as well, as Strigoi who feed on a human who regularly ingests vervain will be burned and weakened upon contact with their blood, which can often be used to the human's advantage. Due to a Strigoi's amplified senses and feelings, their hunger cravings are much stronger and harder to control than the mundane hunger for food that they felt when they were humans, and this hunger will often be connected to and amplified by powerful emotions.

Strigoi are magically preserved in the physical state that they were in when they died, therefore making a Strigoi body a freshly dead corpse, though it is reanimated and protected from decay with dark magic. Strigoi's bodies remain in the exact same state as they were just prior to their deaths as humans (aside from the injuries or effects of most illnesses that caused their death) from an anatomical standpoint, except when it comes to their supernaturally-enhanced abilities and the addition of Strigoi's true face. Their true face is a slightly transformed version of their human face, which includes red, blood-shot sclera with protruding veins around the eyes, as well as elongated fangs. The bite of a Strigoi is so powerful that it can sever the limbs of both animals and humans, as Strigoi are known for having extremely sharp and strong teeth.

Similarly, a Strigoi has human-like physiology, and their bodies continue to function normally as long as they have a steady diet of human blood; for example, their heart still beats to pump the blood they've consumed through their system to maintain their vitality. Because of this, Strigoi can eat human food, though it doesn't sustain their life or provide any nutrition like blood does, and their digestive system will continue to process it and produce wastes just as a healthy human's does. Regardless, many Strigoi continue to eat regular food for several reasons, including out of human habit, to maintain human appearances in order to prevent their exposure, and to help sublimate their eternal hunger for blood and thereby keep their bloodlust at a manageable level.

Strigoi can also become intoxicated by drugs such as alcohol and marijuana just as humans can, although they do have much higher tolerances due to their accelerated healing, which rapidly regenerates the cells damaged by these drugs which causes the sensation of being drunk or high. In fact, most Strigoi frequently drink alcohol throughout the day to help them deal with their perpetual hunger, much in the way that some Strigoi eat human food to help curb their cravings.

Strigoi are immune to the majority of human diseases and illnesses. One exception of this rule is cancer, though Strigoi cannot actually develop it—they have to have had the cancer prior to being turned, as Strigoi blood cannot cure it in humans. These aforementioned Strigoi continue to have cancer even after their transition is complete, as cancer cells rapidly regenerate in the presence of Strigoi blood due to the cells being functionally immortal; normal human cells have "suicide buttons" that allow them to die once their organelles have worn out, but cancer cells have no such button, and Strigoi blood only serves to accelerate the regeneration of cancer cells. The only known substances that can have a serious negative effect on Strigoi are vervain and Werewolf Venom.

Nature

"She is a different person. She's a Strigoi. We're a predatory species. We enjoy the hunt, the feed, and the kill. When the guilt gets too bad, we switch off our humanity and we revel in it."

"You feel like you could do anything. Be anyone. Beautiful things are more beautiful. Everything's heightened, you know? Just a little bit more intensely. Love more powerfully. Anger becomes rage. When you're sad, you're in despair. Grief. Loss. It can cripple you. That's why so many of us turn our emotions off. It just becomes...too overwhelming."

Strigoi have the capacity to be extremely feral, predatory creatures, and can be more beast-like and savage than their human appearance suggests. They growl, hiss, snarl, and curl their lips back when provoked, baring their fangs as a sign of aggression when faced by a threat.

Despite their nature, Strigoi are also capable of human emotions, such as compassion, love, and self-control. However, it's in a Strigoi's nature to be violent, and no matter what their morality or kindness, they will almost always resort to such acts if it's necessary for their survival. While Strigoi typically maintain their human personalities after their transformations (though magnified by the magical properties of vampirism), they are usually dehumanized to some degree, and due to both the violent tensions between hunters, witches, werewolves, and Strigoi (along with the in-fighting within the Strigoi community itself), violence, tragedy, and death of the aforementioned groups and humans caught in the crossfire seem to follow Strigoi wherever they go, even despite their best efforts to avoid it. Many (though not all) Strigoi are inherently selfish and consider other beings, especially humans, to be inferior to themselves, as they see themselves as the top of the proverbial food chain. Even the most compassionate of their kind will often resort to cruelty and torture if it means protecting themselves and their loved ones from danger.

For example, Syd Winchester, despite her generally kind and compassionate attitude, has been shown to be unafraid of killing when the situation calls for it, though she makes it a point not to harm or kill humans unless absolutely necessary.

However, Strigoi experience sensations and feelings much more intensely than humans, and as a result, they have a much more difficult time controlling their emotions, all of which are tied to their hunger in some way. Whenever a Strigoi is overwhelmed by powerful feelings (such as extreme hunger, fear, anger, happiness, love, etc.) their faces become pale, and blood rushes into the veins surrounding their eyes, darkening them tremendously. Their canine teeth will lengthen into fangs, giving them a more predatory look to scare off or threaten their prey and to aid them in feeding on humans and animals. This intensification of emotions experienced by Strigoi also extends to their personalities as well, both the good and the bad aspects of it.

Due to their intensely emotional nature, it is not uncommon for Strigoi wishing to maintain self-control to document their lives after their transition in journals. Though they have consistently shown considerable powers in recollection of specific memories and dates, Strigoi still need to peruse their journals in order to reflect on their feelings and other minute details during certain events. Another function of the Strigoi nature is the ability to "turn off" their humanity in order to make it easier for them to emotionally deal with feeding on (and occasionally even killing) humans to maintain their immortality.

Strigoi who have used this coping mechanism have described the process as "flicking a switch," which essentially numbs their emotions and disables their ability to be empathetic, allowing them to shut out painful feelings such as guilt, grief, and sadness and turning off the part of them that makes them ashamed of their actions. Strigoi with their humanity turned off often think of themselves as purely rational and logical and are not ruled by their emotions. They also have no problem feeding indiscriminately on whomever they please, either using mind compulsion to make the humans forget the experience or killing them outright with no shame or remorse.

Society and Culture

Many Strigoi are nomads of sorts who periodically move around from place to place to make sure their immortality and lack of aging do not lead to their exposure as Strigoi to the general public. Some of them appear to attempt to take on identities within the human world and exist alongside humans (such as Syd, who has moved around a lot in the ten years she's been a vampire) while others instead appear to enjoy living among humans mostly anonymously without making human attachments and then choosing to travel to new places for a change in scenery whenever the urge strikes them.

Attitudes Toward Humans

Strigoi generally coexist with humans due to their shared origins and appearances. Humans are mostly oblivious to the existence of Strigoi, which makes it easier for Strigoi to commingle with their human cohorts without detection or exposure. Most of the few people who do know about Strigoi have either been killed to ensure their silence, compelled to forget their encounters with them, or, in some cases, are compelled not to speak of it to anyone, which lessens the chances that the Strigoi species will be exposed to the public.

Attitudes of Strigoi towards humans vary from individual to individual. Some see humans as simply food, servants, and/or entertainment, while others feel compassion for humans, remembering their lives before becoming Strigoi and not wanting to cause any more harm to them than necessary, and even regarding certain humans as close friends, lovers, or family. In general, Strigoi are known to see humans as less compared to themselves and ultimately regard their lives as being essentially insignificant, as humans are mortal whereas Strigoi live forever. As a result of their immortality and the fatal violence they witness on a regular basis, the death of a human who has no significant connection to the Strigoi is typically a non-event for them, and is either shrugged off as collateral damage or seen as a means to an end in part of whatever the Strigoi in question's current agenda is.

Human Awareness of Strigoi

Only those that consider themselves hunters are aware of the existence of literal Strigoi. In history, there were periods of time where Strigoi were not so secret, but as of the 21st century, the majority of humans regard Strigoi as superstition, fiction, and myth, which is how most Strigoi prefer it to be, as it makes it easier for them to move through the world without detection.