A character in the anime Hellsing. She is named "Seras Victoria" in Pioneer's US dub, and "Celes" in the fansub. I think "Celes" (or maybe "Seles") is a better representations of how it's actually pronounced, and so I will ignore Pioneer in this matter. By the way, Victoria is her last name.
Celes Victoria was previously a member of the elite D-11 group of the London City Police Department. However, D-11 was sent in to kill a vampire who had killed an entire village (turning them into ghouls), along with the regular cops that had previously been sent to deal with him. The vampire wiped out her entire squad, leaving her the sole survivor.
In many senses, actually, she didn't survive the encounter. She had wandered into the church that the vampire used as home, and was about to be turned into one herself when Alucard showed up, sent by the Hellsing Special Institute to kill the vampire (as the police were totally ineffectual against it). In desperation, the vampire took Celes hostage, holding her in from of him like a shield. He figured that anyone working for humans would be unwilling to shoot one. Alucard simply asked Celes (telepathically) if she wished to go with him, and telling her he will shoot through her to kill the vampire if that's what she wants. She says yes, and Alucard does shoot, killing the vampire
After having a 3 inch diameter hole blown into her right lung, Celes wasn't doing so great either. The only way for her to survive was for Alucard to turn her into a vampire himself. And so Celes Victoria became a member of the Hellsing Institute.
Celes has a hard time acclimating to drinking blood as a regular part of her diet. The first several nights, she refuses to drink the bag of medical blood laid out for her, and she also refuses to drink Alucard's blood when he offers. It seems she also has a compunction against shooting anything that is human, making her well suited to be a sniper for the Hellsing Institute, shooting down vampires who hunt the streets of London.
Over time she gets a little more used to her new powers, and starts carring around an 8 foot long sniper rifle (depleted uranium shells: woo!) which she uses to good effect. She also begins to work investigations for Hellsing, walking around in very cute civilian clothes.
Over time, she grows a bit more accustommed to the vampire lifestyle, though as she is quite "young" she is nonetheless often suprised by certain aspects - such as when Alucard uses his full vampiric powers for the first time.
Stylistically, Celes is a very interesting character for Hellsing, because she's completely out of place. For one thing, her bright blue(1) eyes are about 2-3 times as big as anyone else's, which gives her a wide-eyed innocent look at lot of the time. Her sometimes naive actions also offer an interesting contrast to the many secrets held by most of the other characters. Most of the character designs in Hellsing remind me a lot of Trigun, whereas Celes often looks and acts like someone from Dominion Tank Police or maybe even some shoujo manga.
Alucard usually calls her "female officer" (fansub) or "police girl" (Pioneer); the butler "The Angel of Death" Walter later picks up this habit as well.
(1): They turn bright red after she becomes a vampire