"If ever a person feels the need to tell you what kind of person they are, expect the opposite."
This is one of my own personal rules, built off of unhappy experience. This one actually stems from people abusing the concept that first impressions are important. Now, I'm not saying they aren't; they are. That's the problem. Once a person thinks they have a handle on who a person is they stop questioning their evaluation of that person, quite often even in the face of overwhelming evidence.
Many of the more unscrupulous of our compatriots are perfectly happy to use that tendency of their fellow man for their own ends. But first they have to establish that baseline. They have to convince you they're something they're not.
They could try to act like the person they want us to see, but that entails work. Plus, there's also that risk that they might just become the thing they pretend to be. Can't risk that.
Besides, there's a much easier way to go about it. And, for these people, its all about the path of least resistance. All they have to do is tell us who they are. Do that, parrot whatever you say back at you, and their in!
The funny thing is, no one feels the need to tell you who they really are. You'll find that out all by yourself. The only reason they'd have to preemptively inform you as to their character is to short circuit your evaluation process of same. If it happens, start looking for the ulterior motive behind it.
A liar will always tell you they are trustworthy; it's one of the easiest ways to get people to believe in them.
A charlatan will always act as if they know what they're talking about. Ask Bill Gates: a false expertise can go a long way.
A Demagogue will always claim to be fighting for the common man/woman. How else will they get his/her support?
A Nice Guy/Girl will always claim to be an asshole. That's how they try to avoid being taken advantage of.
A Cheater will always claim to be honorable. Lest you actually look for the cards up their sleeve.
Etc.
This isn't to say that there aren't honest people out there. Or those concerned with helping the common person, or assholes, or honorable individuals. There are; they just don't feel the need to broadcast it, because they expect their actions to show it. It is only the person trying to cover future actions that will feel this need.
Note: This also doesn't apply if you go out of your way to ask someone who they are. Most people will answer that question, some honestly, others less so. I personally refuse, telling people to figure it out for themselves.