Not a single entity but everyone! Bitcoin is a decentralized currency and hence the value is not guaranteed by any single entity unlike government currencies. Instead the price is decided by the market following open market trading practice. It follows a simple demand supply rule. Since bitcoin has a limited supply in the market, if demand goes up, the price will be higher. But if the the demand goes down, the price will be lower. Even though the demand depends on various factors, but that's the basic reason for bitcoin's price fluctuation and deciding factor.
Yup! I agree that bitcoin is not controlled by a single person and you can not see any stable price because it is fluctuating and unpredictable, And because bitcoin had a limited supply that is what guaranteeing the value of bitcoin and that is a guarantee that bitcoin's demand will surely increase, And right now bitcoin price had a resistance and dip to the $13,700 value.