How is "素直な" disposition described in English?

I've had a sudden question as mentioned above. And I took a dip into a dictionary, but I couldn't find the definition which I feel suitable to the word.

So I tried describing the meaning of "素直" , which is a Japanese word and one of the noun forms of the adjective "素直な". The following definition is just a personal interpretation.

In my opinion, the Japanese word ”素直”（you should pronounce "sunao"） means the character that makes a person possible to perceive and recognize true feeling of him/herself, and also can recognize both agreeable and disagreeable aspects of one's self calmly. 素直な("sunao na") person has a pure and good heart and so he or she should has a nice friend. But 素直な person might feel difficult about coping with relationship among wicked people. That is, of course, water doesn't mix with oil.

And I think 素直な mind is a key of making one's life a happy one. Could you grasp the meaning of it?