The story of Lisa Trevor is a sad one. You can read all about it in the
Family Picture & Notes file, which describes the architect of the mansion George Trevor, his wife Jessica and their daughter Lisa being held prisoner by Umbrella and being experimented on with the T-Virus. Lisa is the sole survivor, however the multiple viruses on her body has turned her into a hideous practically indestructible monster. She now roams the mansion grounds looking for her family.
Great caution should be taken when you encounter Lisa Trevor. The manacles she wears she uses as a weapon, and it doesn't take many strikes to kill you. After enough ammunition has been unloaded into her body she will collapse, but the battle is not over. Take this opportunity to run for it, otherwise you will literally be there all day.