News today of the death of actor Nicholas Courtney has struck genuine sadness through the geek community and the world at large. A fine actor and a seemingly genuinely lovely man, he was also the longest-serving actor in Doctor Who history.

His character, the unflappable Brigadier Alastair Gordon Lethbridge-Stewart, was one of the most beloved characters in the entire Whoniverse. He made his first appearance in 1969 adventure The Web Of Fear, then a mere Colonel, head of the United Nations Intelligence Taskforce, the international community's line of defence against alien threats. When the series went Earthbound in 1970, the Doctor was employed by Unit and the Brigadier became central to the series' "Scooby Gang".

The Brigadier occupies a unique position in Who lore. It is the Doctor's defining quality to abhor violence, refusing to use weapons even when absolutely necessary. Lethbridge-Stewart was a military stalwart, his instinct to shoot at the alien threat with a catchphrase of: "Five rounds, rapid!" The two men's differing approaches tested as much as complemented each other.

Famously, in the Third Doctor Silurian story, the Brigadier orders the destruction of the underground Homo Repltilia settlement, to the Doctor's disgust. But he also served as a human, and humane, counterpoint to the Time Lord's alien eccentricities, and the pair developed a professional respect and personal affection.

While circumstance dictated that the Doctor's female companions would come and go on a biannual basis, their bromance was a rare constant in the renegade's life, and when our hero returned to travelling through time and space, the Brigadier appeared alongside every subsequent Doctor from the classic series (barring the sixth, unless we're counting the audio-plays as canon, in which case we'll be here all day).

When Unit turned up again in the Tenth Doctor story The Poison Sky, the organisation had toughened up and skewed its moral compass. "Sir Alastair" as he was now known was stranded Peru during the crisis. The Doctor openly pined for the Brigadier's more honourable way of doing things.

His final appearance on the main show came in 1989's Arthurian Legend-riff Battlefield, where the now-retired Lethbridge-Stewart battled a Jean Marsh-portrayed version of Morgan La Fay alongside the Seventh Doctor. The plan had been to kill the character off, but with all hell breaking loose, his death could only have been an incidental plot point, and producer John Nathan-Turner told Courtney, "If you're going to die, I want your death to mean something." This was how significant he had become, and he was allowed to live happily ever after in his country pile. A year later the series was axed.

Sylvester McCoy, who played the Seventh Doctor, said Courtney was very proud of the fact that he was the only Doctor Who regular to appear alongside almost every incarnation of the Doctor.

"I remember when they first told me that they were going to bring him into one of my stories, I was delighted – I was a big fan of his, going back to the Troughton era," McCoy told The Guardian. "I fact, I was over the moon – which, as the Doctor, was a very easy thing for me to do.

"What I liked about him was that he had such humour in his performance. He was playing the "military intelligence", which is a contradiction in terms. But instead of laughing at the character, we came to respect him.That was because of the man himself. He was great company – gentle, caring, a lovely man.

"He'll be much missed. Nicholas was a gentleman in the true meaning of that word."

Lethbridge-Stewart returned to the screen one last time in 2008 for a guest appearance in spin-off The Sarah-Jane Adventures. Now a retired General turned schoolteacher, he was called in to help his old friend. The warmth between the two former colleagues, their lives forever fused together by their experiences with the man in the blue box, was genuinely touching.

It was much the same in real life. Courtney knew he was part of something special, and never stopped enthusiastically cheerleading for Who, attending conventions and taking part in numerous Big Finish audio plays. He was rightly proud to be defined by his defining role. The time-space continuum feels a little bit smaller without him in it.