Like a couple of people upthread I've encountered someone a lot like Trump. Not a family member but my PhD supervisor.
One of the issues is that people who have never had to deal with one of these characters (which is probably the majority of people) simply can't imagine that there are really people who genuinely think and behave like that. They are so far out of what most people consider the human norm, that you simply wouldn't believe they existed until you met one. This is why I think a lot of people thought his behaviour on the campaign trial was all part of a persona put on to win votes.
Since he's taken office and particularly since that press conference it is starting to dawn that, no, he really is like that. It's not an act or a carefully calculated image, that is his true character right down to the core.
He is the preening narcissist. He is the mad king hungry for power and adulation. He is the school bully who throws his weight around not just to get what he wants, but sometimes just because he can.