Hi,

The Face of Boe is Captain Jack and plays a key role in Gridlock and Utopia. Gridlock identifies the four letter secret the Face of Boe was holding (you are not alone) and in Utopia, we understand what it means.

YANA - You Are Not Alone = Professor Yana = Master

Of course, the Face of Boe couldn't reveal his identity(Captain Jack) but knew he had to warn the Doctor. He was important as it revealed (but kind of late) that he is not the last Time Lord because the Master exists.

Note: there could be plenty of more. In the Specials, John Simm (who plays the Master) was spotted filming (whilst wearing a grey wig and face mask to hide his identity). Jenny is also a "Time Lady" and other Time Lords could appear when Steven Moffat reigns.
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