Sergeant Benton (John Levene)
The Doctor first encountered Benton when trying to prevent an invasion by the Cybermen. At that time he was in his second incarnation, and Corporal Benton was under the command of the then Colonel Lethbridge-Stewart.
Curiously absent when the Yeti invaded, Benton appeared again to help the Brigadier and UNIT's recently-appointed scientific advisor (the Doctor) tackle a series of alien invasion attempts and several encounters with the Master. Loyal and trustworthy, Sergeant Benton was a valuable ally, and usually the first at hand to receive the Brigadier's orders.
The Doctor last encountered Benton (now promoted to Regimental Sergeant Major) - and his android duplicate - during the Kraal invasion.