Bonjour Liyunzi,

Jim is correct! "En" is a pronoun and replaces/refers to "the home" (Colette's home) that was mentioned at the beginning of the sentence.

When "faire de [quelque chose] [quelque chose]" is used, the pronoun "en" is used to replace the "de [quelque chose]":

L'écrivaine en a fait un personnage principal -> L'écrivaine a fait de la maison un personnage principal

The writer made it into a main character -> The writer made her home into a main character

I hope this is helpful.

Bonne journée !