Reflections are sentient reflections of ordinary people featured in the sixth Goosebumps book, Let's Get Invisible!, and the More & More Tales to Give You Goosebumps short story, Mirror Mirror on the Wall.
Let's Get Invisible!
Max Thompson found a mirror in his attic. The mirror can turn Max, his brother Lefty, and his friends invisible, but what they don't know is that inside are demons that wants to trade places. Max's reflection almost succeeds, but Max escapes just in time. In the end, it is revealed that Lefty never made it out of the mirror. This is revealed when Lefty, left-handed, uses his right hand to throw a ball.
One day, Bonnie Sue Bower's reflection came right out of the mirror and tried to shove the real Bonnie Sue into the mirror. Bonnie smashes the mirror though and it shatters into a thousand pieces. From the shards rise more and more Bonnies, she then takes out her compact mirror in her pocket and it sucks up all of the miniature Bonnies. A few days after that, Bonnie goes to try out hairstyles in the mirror. But she is pulled inside and traded with her reflection. So now she spends her time waiting for a chance to escape, especially since Reflection Bonnie does not take care of herself and dresses like a slob.
In Goosebumps: The Game, the player encounters a reflection that is inside a mirror while in the attic of Dead House. A book in the study refers to the monster as a "Mirror Demon". The player can defeat it by smashing the mirror with a hand mirror. If the player attempts to smash the mirror too quickly, the reflection will kill you. Once you successfully break the mirror, you will receive the achievement, "I Was your Evil Twin", referencing the Goosebumps Series 2000 book I Am Your Evil Twin.