The secret companion has not been released yet. According the TORHead leak of future patches (which has turned out right thus far!), the secret companion is released in 1.3 -- HK-51.
There is cut content inside game files about him. And BW said people who found it are right, there is an HK 51 planned, and it was supposed to be a surprise.
You can already found a ship on Hoth, with a codex entry about HK 51. It will part of the quest.
To OP : there is no hidden companion. You can't miss them if you do your class quest. You can't reject them, even if you really want to. The only one is HK 51, and he is not released yet.