They are the steemit developers 'steemit inc' in conjunction with all of the delegates we vote for, who run the network in this DPOS crypto currency backed system. One can only change the parameters through a hard fork, which is a rewrite of the code, and must be approved by all delegates (like what Bitcoin is trying to do with segwit and the miners).

No one person has the power because of decentralization, however the collective does have the power, the delegates are our voice. They have power because we elect them.