I find the hardest character to write is one who is sadistic like a pedophile or cannibal. I hate having to get into their psyche.
Funny enough, I rarely write about everyday people. I don't really know what one is. I find boys/men the hardest to write about, but that's not really surprising, being female myself, but I haven't explored all the possibilities; certainly people with disabilities would be hard for me.
It's so hard for me to write about happy characters. I can write to the end of the earth about a sad character. Perhaps with sad characters, I have something to write about. Something happens, they're faced with conflict, and where there's conflict, there's story. But that doesn't mean if you have a happy character you can't have conflict. The conflict would just have to be more external rather than internal. Now that I think about it, I should probably challenge myself & write about a happy character. It'd be a good challenge, a good exercise, and it might do me some good personally.