Most of my favourite characters (both as a reader and a writer) are people I probably wouldn't get on with in real life, if that says anything. My own personal line doesn't have to do with likeability or even believability: it has to do with intelligence and agency. Nothing will turn me off a story faster than a convenient narrative lack of intelligence or an inconvenient emotional lack of it.

For instance, if it weren't for the setting, built-in mysteries, and interesting side characters of the Harry Potter series, I probably wouldn't have finished it, because I can't stand it when protagonists demonstrate a lack of reasonable curiosity or common sense just to make the author's job easier.

Likewise, I actually never did finish Robin Hobb's Farseer series, despite investing in five out of the six books, because I kept banging my head on the wall out of frustration at the emotionally obtuse decisions the protagonist just kept making, to the detriment of himself and those around him. And despite being told from all corners that I'd love it, I really didn't enjoy Sarah Waters' Night Watch because of how the characters' largely unresolved misery sprang from their own poor decisions and lack of agency.

I don't require a character to be smart, but I want to see them at least attempting to proactively engage with their world instead of merely responding to the people and events around them—and if they're not, I want to know why.