I'll only answer the last two, since the first two have already been sufficiently answer.

No, none of the characters are hard to get; you will have to play through again on New Game+ though to get all of them. I know you will have to do at least one play through on NG+ but I don't remember if more is necessary. It's just exploring different paths.

And it's a very good game, it has wonderful music, a great story, and nice gameplay. You should enjoy it.