When he glanced up at the mirror, his mouth said, “Yes, let’s get another look at you.” [loc. 331]
There are a number of problems with this. Firstly, chaos demons are usually passed from Learned Divine to Learned Divine -- not from Learned Divine to clueless teenager. Secondly, this demon has had only female hosts before Pen. Thirdly, the acquisition of a chaos demon automatically makes one a sorcerer: and Pen knows nothing about sorcery. And fourthly... well, fourthly, Pen's body is home to a demon as well as to Pen himself, and nothing in his life so far has prepared him for this.
What makes all the difference -- and what makes this novella so charming -- is that Pen is a decent and compassionate young man. He grants 'his' demon a name -- Desdemona -- and attempts to get along with his unsought companion as best he can. With, it has to be said, some amusing consequences. But by the end of the novella -- after threats to both Penric and Desdemona -- it's clear that the two of them have a workable partnership.
Luckily Bujold has written more novellas in this sequence: they were the perfect post-con pick-me-up.