"The Thing About Cassandra" is a short story by Neil Gaiman. It's featured in the collection Trigger Warning.
Cassandra is the narrator Stuart Innes's first love. At least, Stuart claims she was; he made up Cassandra so that people would stop asking when he would get a girlfriend. He is baffled when a schoolmate reveals that Cassandra got in touch with him.
Tropes for this short story include:
- Cruel Twist Ending: Stuart turns into ash when he touches Cassandra, and she leaves his flat to get ready for work the next morning.
- Girlfriend in Canada: Stuart lies that Cassandra is moving to Canada so that no one knows that he made her up, and no one ever meets her. Cassandra explains that she did move for real, and after her parents divorced she moved back with her mum in London.
- Lack of Empathy: Cassandra is startlingly calm about letting Stuart die by touching her, even telling him beforehand that he isn't her type though he was when they were teens.
- Rule of Three: Stuart hears Cassandra mentioned three times. The third time is when he meets her in person and they get some food after his art show.
- Tomato in the Mirror: Stuart didn't make up Cassandra; she made him up to deal with her parents moving to Canada. He's not a real person but a construct from when she was a teenager.
- What Measure Is A Nonhuman: Cassandra isn't that broken up about letting Stuart turn into dust, though she muses that she could have left him alone and let him wander through life as a shadow of a person.
- Yank the Dog's Chain: Thanks to Cassandra being paid to put on a show, Stuart sells a painting for 75,000 pounds. That's more than enough to settle him for the year, even with the gallery taking a cut. Cassandra then turns him to dust by touching him, and she muses that all the money might fade.
