What is a person called who writes diary?

There isn't a specific word for a person who writes a diary. You could use more general terms like:

* Diarist: This is the most straightforward term.

* Journaler: This term implies a more reflective and introspective writing style.

* Writer: This is a broader term that encompasses many types of writing.

Ultimately, it depends on the context and the type of writing the person does in their diary.