What traits do comic book publishers look in a writer? What should a writer put in his/her portfolio? Thanks you for answering.

Anything comics. It has to be comics, unless you have just a huge, impressive resume in some other kind of creative writing. If you have a serious film or prose background success, you can bring that to the table, but what they want to see is comics. They need to see that you understand the vocabulary and mechanics of panel storytelling.