In many ways, Nicholas Courtney was the most essential actor in classic Doctor Who. From his first appearance in 1965 to his final outing two years ago, he was the most trustworthy friend of that mysterious time traveler from another world.

Courtney died today, aged 81.

His first appearance on Doctor Who was in the 1965 story "The Daleks Master Plan" as Space Agent Bret Vyon, but it was in 1968 that he first played the role that he's remembered for: that of Brigadier Alastair Gordon Lethbridge-Stewart. The Brigadier, as he was usually just called, was a soldier who helped the Doctor to battle alien and other-worldly threats to the human race. After his first adventure, he came to be in charge of an international military organization called U.N.I.T., starting in late 1968.

The Brigadier appeared regularly on the show from 1970 through 1975, and his presence was the single most defining factor of the show's "U.N.I.T. era," in which the Doctor was trapped on Earth and conveniently encountered a larger than usual contingent of deadly threats to 20th Century humanity. The Brigadier frequently seems out of his depth, but never lets that slow him down, whether he's facing plastic dummies come to life, or unstoppable cyborg armies.

It's a tribute to Courtney's acting that the Brigadier became such a beloved figure. Sort of a Commissioner Gordon to the Doctor's Batman, the Brigadier could easily have been a doddering straight man, the butt of the Doctor's jokes or just a source of muscle. But no matter what bizarre situation the Brigadier found himself in, Courtney always found a way to let the character's intelligence and strength of will come through. Famously, at least one real-life Army officer complimented Courtney on how closely he'd managed to imitate the way real young officers behaved. And because Courtney was also an incredibly likable actor, he was able to portray a Brigadier who sometimes resorts to violence even when violence is clearly the wrong answer.

And he's justly celebrated for his deadpan delivery of lines like, "Chap with wings, five rounds rapid." Or: "Just once, I'd like to meet an alien menace that wasn't immune to bullets."

Courtney came aboard Doctor Who at a time when it was struggling, both creatively and in the ratings, and his canny portrayal of the Brigadier helped to make the "stranded on Earth" premise workable. He embodied Earth-bound authority, but also gave the Doctor a regular ally who wasn't just the standard one-dimensional "leader out of his depth" character. The show would have had a much harder time making that new format work without an actor of Courtney's quality in that key role.

The Brigadier was almost killed off in 1976's "The Seeds of Doom," but luckily the script was rewritten to insert a different military figure (who actually survives.) After he left Doctor Who in 1975, Courtney returned a few times — he has star billing in 1983's "Mawdryn Undead" and a pretty prominent role in "The Five Doctors," later that year.

But his real send-off was in 1989's "Battlefield," in which we finally meet the Brigadier's beloved Doris. The whole episode is built to showcase the Brigadier's heroism, and Courtney absolutely makes the most of it. He goes, alone, to confront the Destroyer, who asks if humanity can't afford a better champion. To which the Brigadier responds, "Probably. I just do the best I can." (The good bit is about 4:00 in on that video.)

Since then, Courtney has played the Brigadier in a large number of Doctor Who audio plays, appearing opposite every surviving Doctor up to Paul McGann. He leads a fleet of space rockets to save the planet in the Doctor Who audio adventure "Death Comes to Time." And he wrote two autobiographies, Five Rounds Rapid!, and Still Getting Away With It. He served as honorary president of the Doctor Who Appreciation Society, attended many fan conventions, and always made lots of time for the fans.

His final appearance as the Brigadier was in a 2008 episode of The Sarah Jane Adventures. Sadly, he never got the chance to make a final appearance on Doctor Who itself — his confrontation with Matt Smith's Doctor would have been epic.

Courtney will be sorely missed. [TARDIS Newsroom]