Trope for female lead is they only female character?

Hello, I've been wondering if there is a trope for a female lead that is more or less the only female character (the only example I can think of now is Hua Mulan (the 2009 movie not the Disney one) but I feel like there's lots, how about Mako Mori in Pacific Rim? Okay, she might not be the lead …)?

You want to ask in Lost And Found.

Thanks!