Sil is created by merging human DNA with that of alien DNA. She grows to be what appears to be perfectly normal teenage girl with nothing visually out of the normal. Once she reaches sexual maturity however her alien needs take over her desires to reproduce. She escape the laboratory that's been her home since birth and now she looks for a perfect mate from the human male populace, who are somewhat less robust than Sil as her skin becomes hard and her features alien and her sexual lust puts her in a frenzy that few can survive.
She was later cloned, and named Eve. She too escaped the lab and eventually found another hybrid, to mate with. She did turn against the male hybrid and was killed, but a daughter was born after her death.