The Face of Boe one of the Universe's oldest beings
The Face of Boe comes from the Isop Galaxy and has lived for Millions of Years. He is the species known as Boekind. Him and the Doctor have seen each other in different time lines. We first saw the Face of Boe on Platform 1 watching the Earth die. he organised the event and invited different aliens like Cassandra and the Moxx of Balhoon. He didn't realise that Cassandra wanted everybody there to die and had her Robot Spiders and the Adherents helping her. When her plan was foiled the Face of Boe left which he would meet the Doctor again.
We briefly saw him appear in "The Long Game" when a news report said he had given birth. He has 6 children but all of them only lived up until they were 40 Years.
He later met the Doctor on New Earth where Cassandra appeared again. She followed the Face of Boe there where she hid down under the New Earth Hospital. He sent the Doctor a message using the Psychic Paper which said "Come to Ward 26". He was being looked after by a cat nurse called Novice Hame. He was dying of old age. When the Flesh got out he stayed on his floor which was the only one not infected. When it was over he told the Doctor he would meet him again for the last time. Then he teleported away.His last appearence was when he was opperating the underground lanes as he saved thousands of New Earth people from the Virus called Bliss. He controlled everything trying to protect people from some beasts the Macra. When he asked Novice Hame to get the Doctor she did and teleported him to the Face of Boe. He shrouded her with his smoke to prevent her from getting the virus. He helped the Doctor out by giving him the last of his power. He saved everybody down below and saved New Earth. But his glass broke and he fell out of his machine. The Doctor wanted him to stay alive as they were the last of their species. He told the Doctor one last secret saying You Are Not Alone. Then he died. The Doctor didn't understand what the message was until when he was on Malcassaro he found a Professor Yana who was The Master. The Doctor realised the Face of Boe's message started with YANA. Professor Yana was a Time Lord too.
At the end of Series 3 Captain Jack told the Doctor and Martha that at his university he was called The Face of Boe. That was where the mystery ended.
The Face of Boe appeared in:
Series 1: The End of the World and The Long Game
Series 2: New Earth
Series 3: Gridlock