Brigadier Alistair Gordon Lethbridge-Stewart, while never officially a companion, was the closest friend the Doctor had on Earth for a long time. The commander of UNIT, the Second Doctor was the first incarnation to officially meet and befriend the Brigadier. While he worked more closely with the Third Doctor, it was the Second who started the relationship and it was one that continued all the way to the Brigadier's death (both in the show and in real life) during the Eleventh Doctor's timeline. A true Doctor Who hero.