Well, I was thinking sommit awsum. (And I know the actresses aren't the same person and thusso cannot canonically be the same person).

The Secret Mystery Woman with the Big Shiny Knife that no one knew the name of. eh? eh? eh? *grins* Think of it. Mystery woman comes to Sam's rescue, has profficiant knowledge of how to use a blade, and has a chip on her shoulder called Demonkind. Is blonde, and does not stick around long enough for Sam to catch a name, and probably even recognise her in the chaos.

It's Immortal!Jess! *grins*