Drusilla was a project of Angel, who found her interesting because of her psychic powers. First he drove her crazy by
killing her family, among other things. Eventually he sired her. She then hung around with Angel and Darla.
Not long after that, she sired Spike who became her lover. After Angel left the group, Darla soon left as well, and
Spike and Dru stayed together as lovers for the next 100 or so years.
Around 1997, she was injured terribly by a mob in Prague. Spike then took her to Sunnydale to look for a cure.
After she was healed, she and Spike built The Judge to try to kill The Slayer. During
this time, Angel returned to his evil ways, and to the group. However, due to growing romance between Angel and
Dru, Spike became jealous and plotted with The Slayer to help kill Angel. In exchange for his help, The Slayer
allowed Spike and Dru to leave Sunnydale unfollowed. The plan worked, to a point. Spike had to knock Dru out to
get her out of town. When she awoke, she was very mad that Spike would betray Angel like that and so she left
Spike for a Chaos Demon.
Dru's activities for the next two and a half years are unknown. But in late 2000, she was brought to LA by
Wolfram and Hart to sire a reincarnated Darla. She and Darla joined up for a bit to cause mayhem, but as they never really
got along in the first place, they went their separate ways soon after. Dru decided to go find Spike and be with him again,
who by now had changed very much. He was no longer able to harm humans because of the microchip in his head. Although he
did want to be with her at first, he found that because of his disability, and also his desire to be with Buffy, he couldn't
be with Dru. And so she skipped town, alone again.
Dru has never been much of a fighter, so her vampire abilities have never been fully shown, but she seems quite capable,
and her psychic powers make up for her lack of brute strength. It is interesting to note that when she was injured, her body
would not heal by itself, something that a vampire normally would do. It is possible that she was injured by a spell which
made her weak, or some sort of vampire poison. Another interesting note is that Dru passed out when she was strangled by Spike.
Why this is, is anyone's guess. Vampires don't need oxygen, and so blocking the flow of blood to the brain or strangling
her would be pointless and not harm her. This is obviously a mistake in the writing, but one could say that Dru is so crazy
that she passed out for psychological reasons.