Alex Klein

Alex Klein (PROSE: The Time Travellers' Almanac) 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, Doctor Who: The Encyclopedia and The Time Traveller's Almanac gives his full name as Alex Klein. Furthermore, Doctor Who: The Encyclopedia reveals that Alex Klein is a UNIT operative.