An 'imaginary' friend who you miss but don't want to forget. Usually she but to some it could be a he.
boy asks wheather he can use the empty chair next to you.
you reply with "no flo is sitting there"
In the car going to your friends house:
"shit mum, flo. shes coming too, i was supposed to pick her up"
mum does not question flo and replies with:
"oooh no, do you want me to go back?"