I don't know if there is such a person around you, who is obviously single, but uses the portrait of a couple, or who is obviously single, but often sends some ambiguous content in the circle of friends, so that people who don't know the truth misunderstand that she has a boyfriend.
Some girls are single and use couples' heads, probably to block peach blossoms for themselves. This kind of girl has no plans to fall in love at present, but there are many suitors around her, and many people in her circle of friends care about her lifelong events. Girls use couples' heads, probably because they want the opposite sex who always bothers her to stay away. Maybe she lied that she had a boyfriend when she refused these people.
Some girls are obviously single and use couples' heads. Maybe they just think this head is cute. Maybe girls don't even know it's a couple's head. She just thought she liked the style of her head portrait, that's all.
Some girls may use couples' heads with good friends or close friends. Girls' friendship is sometimes closer than we think, and girls occasionally joke that they call their girlfriends husbands; This is a little fun between girls. Girls and girlfriends use couples' heads because of good relationships. The couple's head portrait itself is made by netizens, and it is not stipulated that only people who have been in love are eligible to use it. So if the girl you like uses a couple photo, don't be sad yet. Maybe she is still single.