Idle thought of the day, caused by hearing the audio commentary for the DW season 4 episode Midnight, in which David Tennant, RTD and Alice Troughton praise young Colin Morgan - who plays Jethro in Midnight - and correctly predict he'll be a hit with Merlin, plus seeing the "Et tu, Jethro" moment when Jethro gives in to the lynch mentality:

1.) Midshipman Frame, aka Alonso, from Voyage of the Damned
2.) Quintus, in Fires of Pompeii
3.) Sgt. Ross Jenkins, from The Sontaran Strategem/The Poison Sky
4.) Luke Rattigan, same episodes as Ross
5.) Jethro in Midnight

= secret subtext of season 4: Doctor bonds with good looking dark haired young male sulky adolescents and/or young servicemen? And here people claim he has only eyes for female blondes!