Once Usagi's close enough, the identity of the harpist becomes all too clear: the planetary sigil marking her forehead cannot be faked. She looks different, with the dark hair, but it's so clearly her.

Mercury.

As soon as she realizes someone has approached without announcing themselves, the music stops and Mercury stiffens. Her eyes seek out any movement, her ears strain for sounds.

"Who's there?"

And that's when she sees Usagi - no, the princess. She's on her feet, posture defensive, the harp no longer pretty music but a potential weapon in her hand.

"What have you come here for?"