What hidden character? If you mean the figure that's grayed out when you check your team status then you'll have to complete all companion missions first. Then you'll be told by the IE that an IFF module is available. You then launch on that mission to acquire the IFF and you will meet the hidden character on board the ship (containing the IFF). Once he boards your ship (at the end of the IFF mission) you must speak to him for him to join your crew. Then you will be launched on his companion mission.