Is it possible to say about somebody who is ready to help and does not hide his emotions an “open person”? What would be the opposite to the “open person”?

Dear Vika: some people are "stolid" meaning they show little emotion, but may not even be hiding any.
British upper class members pride themselves on being reserved and having a stiff upper lip, and feel superior to people who reveal their emothons. Yankees pride themselves on understatement, and in general are not comfortable with emotional displays. I know what you mean by an "open person" but perhaps some of our other members might have suggestions for improvement on that. At the moment I cannot think of one. Bill

Hey Vika - to call such a person an "open book" might be more appropriate, although that phrase does not also encompass the meaning of being ready to help. It merely suggests that such a person is easy to read - he hides nothing.

You might say someone who shows his feelings easily "wears his heart on his sleeve." That, again, simply means he hides nothing - he shows all that is in his heart.

I can't think of any slang or vernacular expression that means both "ready to help" and "doesn't hide emotions" at the same time. O, wait a minute - you might say someone is an "eager beaver." That generally connotes someone who is very willing and ready to help, and may also be very open and even a bit naive. I always think of "eager beavers" as being people who may be easily hurt, because they are so eager and genuine, and they rush in where angels fear to tread.

Let us go in peace to love and serve the board.