This is the person with whom you reunited, for whatever reason, after previously ending a relationship. She was your ex-girlfriend and now is your y-girlfriend. The term was created to facilitate communication. The inspiration for this term rose from the Generation Y following Generation X (“Gen X”). The homonym was appropriate for the reasons everyone asks.
“My y-girlfriend and I are going to her cousin’s wedding.” The listener can understand how that experience may be different than if you were a new, current, or just a friendly date. In general, the listener understands that you have had a relationship with this person, parted with them, grown, and reunited after working out your problems….or not, so some say, “Why?”