I dated a girl like that.
Well, to be clear, I thought it was on-again, off-again, and the realization I was a side piece was just one of many lies revealed at cruelest tempo.
Suffice it to say I dated someone completely made up in her mind and about which she invested surprisingly little.
I sent her postcards of condolence on the anniversary of the night we finally broke up, consoling her on the death of [pet name], for a few years. Until I realized I was still more invested in the bit than she was, and I learned she now has 4 kids with different surnames each.