Gordon Freeman, the Silent Protagonist of the Half Life Trilogy (That Never Finished) is secretly the Rani. How do I know this...
Freeman never speaks. Why is this? Obviously in case one of the Scientists in the Black Mesa facility is the Doctor. If he speaks, they might realise he is in fact the Rani.
He continues this is HL2 in case the Combine find out he is a Time Lord and try to get his TARDIS (The Crowbar).
Other Reasons Edit
The concept art of Freeman for the uncompleted Half Life 3 shows this.