Alex Klein

Alex was the senior aide to Harriet Jones in Downing Street in 2006. He was with her when she was teleported aboard the Sycorax ship. He acted as a translator until the Doctor's TARDIS resumed its translation.

After giving Torchwood One Jone's permission to murder the Sycorax, the Tenth Doctor whispered to Alex, "Don't you think she looks tired?" — six words that began a chain of events leading to Harriet Jones' downfall. (TV: The Christmas Invasion)

A Sycorax scribe recorded the events of the invasion, including Alex's actions in the Sycorax ship. (PROSE: The Final Darkness)

Behind the scenes
The character is only referred to as "Alex" in The Christmas Invasion, however The Doctors Revisited and Doctor Who: The Encyclopedia gives his full name as Alex Klein; the Wiki uses this name for disambiguation purposes, however, as it has not been used in a valid source, it cannot be used in the in-universe portion of the article. Furthermore, Doctor Who: The Encyclopedia reveals that Alex Klein is a UNIT operative.