Quasi Girlfriend
A girl who by all appearances, actions, tastes and adventures is your lover, yet is unable to commit to being your girlfriend at the moment.
Jane was Tim's "quasi girlfriend." Jane was afraid of committment and losing Tim as a friend.
Quasi-girlfriend
A girl who has a boyfriend / husband who is not present in the physical that you date / have sex with / are in love with, until the boyfriend / husband returns.
Meghan was his quasi-girlfriend; She was only seeing him seriously until her boyfriend returned from Germany.