Surprisingly, no one. Rather, Bitcoin is controlled collectively.
Bitcoin is totally supported by the free market. The best idea wins. Anyone can view, use, or modify the software. It's open-source. All transactions taking place on the Bitcoin network are publicly viewable and can be independently checked by anybody for validation.
Some might wonder if the developers that created and work on the software control Bitcoin? They don't. The only power developers have is the ability to write software compatible with Bitcoin. It's up to users to decide to use it or not. If there is a change made that people don't agree with, like adjusting the limit of 21 million bitcoins, people can refuse to upgrade. As long as a majority of people are in agreement about how bitcoin works, it will continue working that way for those users.