The White Guardian?

The Doctor and Romana have finished recovering the segments to the Key to Time, an all-powerful artifact, and are greeted by the White Guardian, who sent the Doctor on his mission at the start of the season and now asks to have the Key returned to him so that he may restore the balance of the universe. The Doctor however, notices that the White Guardian is uncharacteristically dismissive of the fact that the sixth segment was Princess Astra, a human being, and realizes that he's actually the villainous Black Guardian in disguise.

Example of:
Out-of-Character Alert