Pop quiz: What’s wrong with this sentence?
The loud man in the two-toned shoes and plush moose hat asked if anyone had seen the shy blonde that he had been romancing all evening.
Other than the plush moose hat, where did I go wrong with this sentence?
Answer: I used “that” to refer to the shy blonde.
In the English language, we never use “that” to refer to a person. That refers to penguins, bison, Venus fly traps, and hot-air balloons. In other words, that indicates anything that is not a person.
On the other hand, we use “who” when we indicate a person. Who refers to the mailman, the trombonist, and to Lottie’s Latin lover. In other words, who indicates anyone who is a person.
“But Kate!” you cry. “I have a vampire/changeling/alien in my book! He/she/it isn’t a person! Which do I use?”
I would say that that depends on how you want to present said creature. If you want your vampire/changeling/alien to be on the anthropomorphic side, then use who. On the other hand, if you want your supernatural creature to come across as more beastie than human, then use that.
The difference is subtle, but by using this one word correctly, you can indicate to the observant reader how, exactly, to view your character.