Q: How important is it that the main character be likable at the beginning of the book?
A: It is very difficult to engage readers if your protagonist is unlikable. Difficult, but not impossible, as Scarlett O’Hara demonstrates.
If your goal is to start out with someone who is unlikable but becomes heroic—like Scarlett, who starts out as an obnoxious bit of work—then it helps if you can show a glimpse of the heroic early on. High ideals, generosity, courage…something positive. It only needs to be a line or two, maybe in her internal monologue or in something someone says to her. It’s very important to give the reader a reason to care about her or at least to sympathize with her.
And that’s the key. The character need not be likeable as long as she’s someone we can feel empathy for. So another trick for introducing the unlikable protagonist is to put her in a situation the reader can sympathize with or relate to. When we meet Scarlett, for example, she’s under the threat of impending war. Although she’s very flippant about it, we know she’s in for trouble.
Another famous unlikable protagonist is Ebenezer Scrooge. Dickens cleverly compensates for Scrooge’s nastiness in two ways. First, he provides a narrator who lightens things up with some humor. Second, he gives Scrooge a likable sidekick, Bob Cratchit, and of course the extraordinarily sympathetic Tiny Tim.
So yes, it is important that the protagonist be likeable, but as with so many things in art, it isn’t a requirement so much as it is a guideline.