Boy writes story, interacts with his alter ego

Question

Publishing language: Unknown.

I read this book in school many years ago, then in my native language: Swedish. Likely it was originally in some other language.

What I remember:
The protagonist is writing a story where the main character (his alter ego) talks to him directly. Later the boy's sister writes, which results in her alter ego appearing in the story. It is set in "modern times", since I remember the boy writing parts of the story using a modern day computer.

The book had illustrations but I do not remember it being a graphic novel.

Answer 1

This is *A Villain's Night Out" by the late, great, Margaret Mahy. The blurb on my library's catalogue says:

In Squidgy Moot, Formby has dreamt up the perfect villain for the book he plans to write. But Squidgy immediately appears as large as life, and he has his own very decided ideas for the story.

I remember the pupils have to write stories to fill the shelves in the school library, as the teachers have spent the book money on a coffee machine for the staff room. Published in 1999, illustrated by Harry Horse.