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".

As was noted the manga covers this in detail and part of this is covering how King rose to the position he currently has in the Hero association in chapter 38.

In this chapter it is revealed that he has a tendency to attract problems and monsters and that in the end he is rescued by Saitama. During one of these events he gets the scars on his face. Because of his physical appearance and being at the scene afterwords so often he is associated with saving the day. Because of the money he gets from that perception he does not bother to correct it.

Also if you read the one punch wiki article for king it explains some more about his character creation that will help explain what is going on

King was created to be the opposite of Saitama where he is weak but looks powerful.

King is actually weak. He is like Saitama's counterpart: Lucky but not strong. Whenever Saitama defeats the monster in one punch King aleways seems to find himself nearby. His firm posture and ability to stand in front of monsters without outward fear makes him seem like a strong character however, his true self is not powerful at all.

In Chapter 37 and 38, King tries to hide away from the monsters and went Saitama has dealth with them King is associated with such a high status and power.

King is likely weak/of average human strength

First of all, there are three "sources" to draw from, none of which is really more "canonical" than the others since ONE had a hand in all of them:

• Manga (drawn by Murata, story by ONE)




• Anime (based on the manga by Murata/ONE)




• Webcomic (drawing & story by ONE)

In all three sources, we have yet to see King actually engage in a fight, so it's still possible for ONE to throw a curveball here.

That being said, there's a number of ways we can infer his actual strength:

1. 
   

   His past self

   
   
   
   
   
   • The scar on his face was put there by a Tiger-level monster[1], some time in the past (max two years ago, since he was saved by Saitama after he started his hero hobby, but before he lost his hair), so his past self was at least weak enough to be permanently scarred by a Tiger-level.
   
   
   
   


2. 
   

   Self admission[2]

   
   
   
   
   
   • It's possible he's just underestimating his own strength, but unlikely
   
   
   
   


3. 
   

   He can't even unsheathe a katana[3]

   
   
   

   
   

   He tries for a good 2+ minutes to unsheathe Atomic Samurai's katana, and fails miserably, and his arms are visibly shaking/exhausted afterwards

   
   

   
   

There really isn't much material, though, since he continues to miraculously anything that would actually showcase/prove his strength/weakness

For the "King is actually strong" conspiracy theorists, though, there are some tidbits floating around:

1. 
   

   His physique

   
   
   
   
   
   • Games all day everyday, and yet somehow doesn't look like Pig God, but also avoids looking like a bundle of dry sticks[4]
   
   
   
   


2. 
   

   His brief encounter with Garou

   
   
   
   
   
   • In the manga, "the genius" Garou is able to predict an opponent's movement, and fight/counter accordingly. Looking at King, he somehow predicts various evasive actions, rather than King just sitting there, unable to respond (as he would if he were actually weak)[5]
   
   
   
   


3. 
   

   Expert climbing?

   
   
   
   
   
   • At the beginning of the Alien arc, Saitama busts a hole straight upwards through the Hero Association HQ's ceiling. In the manga, King is shown emerging from this hole at the top of the farily tall HQ building, unassisted, without breaking a sweat. This would suggest at a minimum an above-average level of physical fitness.[6]
   
   
   
   

Sources:

[1] Manga Chapter 38, page 15

[2] Manga Chapter 38, page 12

[3] Webcomic Chapter 109

[4] Manga Chapter 48, page 21

[5] Manga Chapter 77, page 27

[6] Manga Chapter 31, page 46