Sil is an alien/human hybrid that was developed and raised in a laboratory environment. She was created by the Search for Extraterrestrial Intelligence program.Technically she is not a true villain in the sense of being "evil" but is rather an amoral and highly dangerous life-form that seeks to mate with a suitable human at any cost; human life is meaningless to her and as a result she is extremely violent and murderous by human standards. Due to her childlike lack of understanding Earth based cultures, Sil can be viewed as rather naive, just don't piss her off.
Known as test subject S1L ("Sil"), she was formed from the fertilization of a human ovum with alien DNA. Sil was raised and monitored in a controlled lab environment until the scientists studying her attempted to kill her. Escaping her confines, she broke out into Los Angeles where she evolved and fell to her alien instinct to find a suitable male and mate; becoming an intelligent but hyper-sexual predator.
Sil has human-like emotions, including, to some extent, a desire to be loved and accepted, as well as having an emotional attachment towards her offsprings. When she kills, she does so out of instinct, out of anger and jealousy or defending herself from getting hurt.
Biology and Abilities
Her alien form is chitinous and reptilian, but still humanoid in its appearance. Her "hair" is a mass of prehensile tentacles which are slicked back behind her head when not "active".
She possesses two sets of teeth with the internal set being razor sharp. Her tongue is tipped with sharp spines or barbs and is her primary defense mechanism (or weapon) when with a potential mate. When threatened, she can impale her aggressor with her tongue, usually during kissing, by inserting her tongue into the victim's mouth from within which she can stab into the victim's skull or out the back of their head, resulting in her infamous "kiss of death".
Her breasts, rather than storing fat or mammary glands like a human female's, instead store additional prehensile tentacles which can emerge from her "nipples". She can use her breast-tentacles as weapons (wrapping one around her victim while the other penetrates their mouth and chokes/stabs them to death) but they can also be used in her species' mating rituals.
Sil possesses sharp spines that run up her back, she can retract and extend these at will.
While capable of conceiving children in her pubic region like a typical human female, Sil is shown to give birth via her chest cavity, which splits and opens to allow a fully-grown (or still-developing) infant to emerge from her.
Sil also possesses the ability to rapidly regenerate from any injury, from a broken scapula to a missing limb. However, she appears incapable of regeneration if her body is destroyed beyond the capacity for regeneration, such as being burned to death.
According to the third film, Sil is also disease-free, possibly boasting an immune system that is superior to a human's.