Who is the person offering to help Buffy?

A witch

"Who are you?" asked Laurel of the demure young woman who had just now offered to help Buffy.

"My name is Amelia Madison," said the woman. "I believe I can help your dragon slayer but I'm going to need immunity from the Watchers Council."

"Why would you need immunity from the Watchers Council?" asked Laurel.

"Because she is the daughter of Katrina Madison," said Sir Roderick. "One of the most infamous witches that ever stepped foot in the British Isles. I should know. I was there when they executed her mother two years ago. Apparently she had the bad luck to try and cast a spell on a dragon slayer and it all went downhill for her from there."

"Well, for what's it's worth," said Amelia, "I'm nothing like my mother. I try to use my powers for good, not evil. The very fact that I'm revealing myself now instead of staying quiet should tell you something."

"How do I know we can trust you?" asked Sir Roderick.

"How do I know that I can trust you?" asked Amelia.

"Well, somebody's got to trust somebody or this whole town is going to burn," said Laurel. "What do you have to lose, Sir Roderick? Do you really want to lose your best dragon slayer because you choose this moment to indulge in the sin of pride?"

"Very well," said Sir Roderick. "If you can help out, Amelia, you have my word as a gentleman that you'll receive immunity from the Watchers Council. Now what is your plan?"

Amelia said, "Well, I'd tell you but it would probably be a lot easier just to show you."