A demon that writes messages on your mirror with blood but they’re useful messages. Like “remember you have yoga at 6 tonight”
You will enjoy this story: http://www.drabblecast.org/2014/08/17/drabblecast-335-whatever/
(A guy leaves notes for the creature that lives in the walls. The creature leaves weird runes burned on the table and replaces the coffee creamer he used up. They watch the Oscars together. It’s a good time.)