Octavian (The Time of Angels)

Father Octavian was a Bishop Second Class who had deployed Doctor River Song on a covert mission which led to her presence on the Byzantium, that housed a Weeping Angel.

Biography
When Octavian arrived with his clerics at the crash site of the Byzantium, he was displeased with River, until she introduced the Doctor, the "equivalent of an army" she had apparently promised for assistance.

While discussing their plan of action, Octavian questioned Song whether she had informed the Doctor of her true identity.

He led his men into the Maze of the Dead below the Byzantium, three of whom were soon killed by the Angel. The Doctor assured him they could not have been saved, but Octavian reminded him that he would be the one to tell their families while the Doctor went off in his blue box. Nonetheless he still told the Doctor he had faith in him when they were surrounded by Angels. (DW: The Time of Angels)

After entering the forest vault, he pretended that he and River were engaged to accompany her and the Doctor to the flight deck. On the way he revealed that she was in his protective custody. At the flight deck, when River had already gone through, he was attacked by an Angel. With the Doctor looking at it, it was frozen, trapping Octavian in a headlock, before it could kill him. He revealed to the Doctor that River killed a man who was "a hero to many", but whose identity he feels the Doctor emphatically "[doesn't] want to know", and not to trust her. The Doctor refused to leave him, but Octavian told him to go, glad that his courage did not desert him in the end. When the Doctor stated he wished he knew him better, Octavian replied that the Doctor knew him at his best, and he was presumably killed by the Angel as the Doctor left. (DW: Flesh and Stone)

Behind the scenes

 * Several of Octavian's comments imply that he has met the Doctor before. When River "introduces" the Doctor to him, he is satisfied that he is the equivalent of an army. Later, he asks River if the Doctor knows who she is yet, implying he knows something River did that would change the Doctor's views on her. Also, when the Doctor tells him that he wished he knew him better, Octavian said he thinks he knows him at his best, implying that the Doctor would later meet him when he was not as good a man. Conversely, "you know me at my best" implies no need for the Doctor to have known Father Octavion better, since he was witness to the zenith of Octavion's character.