Seras was originally a police officer who, along with the rest of her squad, was sent into the village of Cheddar, where a vampire disguised as (or possibly was formerly) a priest had taken control of the town and turned its inhabitants into ghouls (zombies). She was taken hostage by that priest vampire, who used her as a shield from Alucard, she was then given a "choice" by Alucard after shooting her through the lung in order to kill the vampire: she could either die as a human, or be turned into a vampire herself, and live. She chose to live, and so Alucard turned her into a vampire, her female virgin blood allowed her to successfully make the transition from human to vampire (non-virgin or same-sex vampire victims become ghouls- this aspect is only present in the original manga and the OVA series however; in the televised series, vampirism is simply determined by the will of the drinking vampire, and possibly by the victim being willing to make the change, and vampires can raise any newly dead humans nearby as ghouls). She was employed by the Hellsing Organization immediately afterwards, and it supplies all her needs such as sufficient blood for consumption and shelter secure from any daylight exposure. Seras is a strong willed woman, and apparently had always been since her childhood which is confirmed through flashbacks.
Seras Victoria is often considered a tomboy; a very brave, strong-willed and heroic young woman who often rejects following orders of her master that cross with her personal beliefs. However, she is a loyal and trustworthy soldier who will diligently follow the orders of her commander, provided said commander has proven his or her worth. Her level of maturity in comparison to the other characters is somewhat questionable; she seems to be the child the others could not be, yet possessing a sort of worldly maturity and superficial intelligence that most other characters lack, as well as a sort of 'nature winning against nurture' mentality, as opposed to most of other characters