Facilinatrix
A facilinatrix is a girl who’s there to help the main girl out and make sure the other guy doesn’t get all the fun. She might even take some of the punishment herself if it makes the main girl happy.
'She’s the one who takes the blame so I don’t get embarrassed.'
'She’s like the best friend who’s also the babysitter.'
'She’s the one who lets me know when I’ve gone too far.'