In One Punch Man, is King actually weak? In One Punch Man, King seems to be running away from fights. He does not seem to show power but only firmness. I want to know if King was just pretending that he is powerful or if he actually is.
I'm not sure how you missed this if you've read the manga. As mentioned by people in the comments, this will be eventually revealed, specifically starting at Chapter 37. 

In this particular chapter,


   instead of helping Genos and Saitama, King went home. It was also depicted how he was afraid and how it was by chance that he was near the locations of defeated villains, resulting to the misconceptions that he is "The Strongest Man on Earth".