Who is this mysterious person who is blocking that exit?

Spike

"Get out of my way, Spike," said Buffy.

"Not a chance, luv," said Spike. "There's no way I'm letting you open that door again as long as the sun is up. And you're definitely not leaving this basement."

"But your girlfriend just threatened to kill me."

"Well, that's Dru for you."

"No, seriously. Your girlfriend just threatened to kill me!"

"Well, she does like to talk a big game but in the end --"

"She already killed one vampire slayer, Spike! Now she wants to kill me!"

"Dru killed a vampire slayer? Oh, right. I forgot. Good for her."

"What part of 'your girlfriend wants to kill me' don't you understand, Spike? And can you please look up here at my face while you're talking to me."

"I'm sorry, luv. But I don't meet too many Slayers dressed the way you are today."

"To heck with it," said Buffy, and she reached out to push Spike aside.

Spike, of course, pushed back. With all thoughts of modesty cast aside, Buffy punched back. Then she kicked Spike in a place where he was still quite human and pushed him away, only to feel a quick tug on her blonde hair. And then suddenly her neck was being held in a chokehold.

"Not so fast, my little naked one," she heard Dru say from behind her. "Mummy still wants to have a little talk with you."