Alex Klein

Alex Klein was a UNIT operative assigned to assist, (PROSE: The Time Travellers' Almanac) and work as senior aide to Harriet Jones 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 Jones' 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) After Klein's UNIT debrief, word quickly spread that the Prime Minister's health might be in question and, before long, a media and political storm forced her from office. (PROSE: The Time Travellers' Almanac)

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.