A person that is only desired or wanted at their significant other's conveinience.
From a quote by "Claire" in the film Elizabethtown
The significant other of the substitute person may not "have enough time" or "be too busy" for a relationship, or they may only be interested for the purpose of sex, money, status, or general need of someone to talk to during times of stress. The substitute person generally won't get the emotional fulfillment from the experience that he/she may crave, and will feel unimportant yet somehow will continue on out of hope.
This can only end in tears for the substitute person.