I put essential but there are a very few exceptions but they are in the major minority. I’d say especially with IF, a self insert in the story is very important to me. I like being able to fully immerse myself in the story and imagine events are actually happening to me, something you can’t do if the story has set characters. If I wanted to read about a pre-established character with a set name, gender, backstory etc. I’d most likely just pick up a regular novel, it’s just one of the reasons why I prefer the IF medium to regular stories.