"Laura Hellsing" is in reality a Baobhan Sith – a Celtic vampire often referred to as "the White Woman". Baobhan Sith often wear long green gowns, and have been known to use hypnotism to ensnare its victims. This particular one had stabbed Integra in the stomach before drawing her blood... then Alucard (because of Incognito's boasting) realized what was happening, interrupted and slew the beast before Integra was killed. And as a result, Walter was released from the enchantment that forced him to restrain Seras and both raced to Integra's aid.
However, because she was bitten, Integra nearly kills herself by stabbing herself in the neck to draw out the creature's taint from her body. She was later saved in a hastily-arranged operation soon afterward.
Baobhan's name is pronounced buh-ban she.