The Psion

From the introduction:

The Psion was designed to be a more complex, reactive class than the others. Specifically, it deals almost exclusively in reaction and interrupt powers. By priming themselves with one of several powers during their turn, the Psion gains access to these versatile options to be deployed at any point during the round. Note that these reactive choices require you to keep a keen eye on the entire battle and, like the Summoner, this class is designed for players who enjoy that level of control and won't be paralyzed by so many choices.