Sil

Sil was portrayed by Michelle Williams, when she was a child, and was played by Natasha Henstridge, when she was a young woman, in the film Species.

Species
She was created by human scientists from NASA on Earth under the leadership of Dr. Xavier Finch with the help of an alien DNA formula they received from space through a response to SETI (reply to messages sent to space by humans since 1979). They mixed this alien DNA with female human DNA under the assumption a femal would be more docile and this led to her creation.

She is an alien human hybrid that grows very fast, is extremely strong and capable of healing its wounds extremely fast. In only three months, she already has the height of a child. She grew up in isolation and was considered a threat by them in time because of obscure behaviour, while she was asleep and therefore, they decided to kill her.

However, she was able to run away in the nick of time during the murder attempt by breaking out of the cage she lived in and then running away from the place. After that, she boards a train and flees to Los Angeles.

During the voyage there, she grows up in a sudden metamorphosis and becomes a young woman with an extreme intelligence, but she also shows ruthlessness by killing two on her way there. Having now reached an age where she can procreate after that, she only thinks about that and acts ruthlessly towards that goal to the extent of even killing for that purpose. When she realises she is being hunted by the scientist who created her and by allies he recruited for that because of the threat her procreation instincts mean towards the existence of the human race, she manages to fake her death in their eyes by making them believe she died in a car crash and by leaving part of a finger she removed and which grew again at her body at the crash site.

After that, she pursues her goal to procreate again and changes the colour of her hair in order to make it black instead of blonde in order to appear incognito. Then, while they celebrate her "death" at a hotel, she succeeds in her goal by using later a team member there as a breeding partner without him knowing about it until it is too late.

She kills him after having sex with him and flees showing her alien body on the way, too, because the other members of the team members find out she is alive through their empathic member Dan Smithson, who feels her presence and her actions. They realise this way what she has been doing and begin to hunt her down to the extent that they even go to the sewers under the hotel, where she went after that.

There, during a fight to the death, she kills another team member, the one responsible for her creation. She also puts the rest of them at a deadly danger to protect herself and her child. Still, they manage to kill the child through Smithson by burning him with a flamethrowers she in the meantime had at a tremendous speed and later they also kill her by shooting her with a grenade launcher, which also blew her head off, something which was done by Preston, the killing expert of the team. After that, her body falls into a place oil full of fire created by the death of her child that fell into the oil and burns into ashes.

Species II
An embryo of hers was cloned by a government unit led by former hunting team member Dr. Laura Baker and the result of the cloning was a new alien human hybrid called Eve, who looked exactly like her.