Senshi. Either this girl is acting Mercury's part far too much... or she was referring to Joan. In her actual state.

She still has her own speed. She's become a senshi herself.

"Same reason people from other worlds show up."

Which is to say, sometimes it happens. As it all happens, the taller senshi still does not raise her guard against this Mercury's own. If it is a civilian, she was playing around. If it was Ami, Joan would tell her not to point that thing at her.