How to explain “闷骚” (mēn sāo) to English speakers

There is a Chinese buzzword called "闷骚" (mēn sāo). It refers to people who look quiet, cold or even dull outside, but inside they are volatile, charismatic, hot and sexy. It may also be used to describe a person's duplicitous personality, but mostly in the eulogistic sense. Someone translated it into "surprise package." I'm wondering if there is another common used phrase for it in English-speaking countries. 

汉语里有一个流行语叫“闷骚”，它是指外表冷漠甚至迟钝，但骨子里却热情如火，极具个人魅力，也很性感。它也可以用来形容一个人的个性很具有欺骗性或迷惑性，但这种形容很多情况下是褒义的。有人把它翻译成了“surprise package”。我想知道在英语国家到底有没有更常用的短语来形容“闷骚”。

I can't think of an expression that directly applies as a noun phrase for this type of person. May I suggest some phrases that refer to or describe a person to see if they convey the same idea?

我想不出来一个专门的名词短语，用来直接描述“闷骚”的人。但有些短语可以用来形容此类人，不知道是否可以表达同样的意思。

Possible the expression "he/she is like a smoldering ember", it indicates a person who has the potential for passionate feeling or action but has not yet burst into flame.

也许可以用这种说法：他/她就像死灰复燃。意思是说一个人很具有潜力拥有热情似火的情感和狂野的行为，但还没有发展成“熊熊烈火”。

The other term that comes to mind is "still waters run deep" which implies that people who are quiet and don't try to attract attention are often more interesting than people who do try to get attention.

我另外想到的短语还有“静水流深(大智若愚)”，用来形容一些沉默寡言的人。他们不喜欢哗众取宠，但却经常比喜欢哗众取宠的人更有趣更受欢迎。

Are we "beating around the bush" here? Does this Chinese expression refer specifically to a hidden sexual passion? Some of the responses seem to indicate this. Or can this be a hidden passion of any kind?

我们谈论这个话题时是不是有点绕圈子？这个中文流行语是来具体形容一种藏在内心深处的激情和欲望吗？有些回复好像是这个意思。或者说它用来形容一种隐含的激情？

You have a good guess! In some cases, it does refer to a hidden (sexual) passion, but not very specifically.

你猜得不错，有些时候的确是用来形容一种隐含的欲望和激情，但并非专门用来隐含这种意思。

So it seems that "still waters run deep" is the very phrase to describe a "闷骚" person. 

这么看来很难找到一个能完全形容“闷骚”的固定英语短语。最接近的短语也只能是“Still waters run deep(静水流深/大智若愚)”了。