Dead Partner

So you're watching a Buddy Cop Show. The Naive Newcomer on the force has made his way to the precinct, ready to fight crime and turn this city on its head, according to the chief, he's about to be paired off with the grizzled old, potentially loose cannon of a cop. You see, the Grizzled Cop doesn't currently have a partner, and will protest having to take on the rookie because, as he will vehemently and adamantly bark at the chief, "I WORK ALONE!"

Cut back to Grizzled Cop's desk, and pan to a picture of him and another boy in blue, perhaps as Naive Newcomers themselves. Then pan over to the desk paired off with his, completely empty. This is the camera telling the viewer that, in fact, the cop used to have a partner, and now he's dead.

Usually used as a form of Death by Origin Story, and frequently as a result of Grizzled Cop's loose-cannon ways. Compare Dead Sidekick and maybe Dead Little Sister.

As a Death Trope, Spoilers ahead may be unmarked. Beware.

Examples of Dead Partner include:

Anime and Manga

In Legend of the Galactic Heroes, when Yang's old friend and mentor figure of sort Alexander Bucock gets killed during a desperate battle he has started to give Yang enough time to organize his own troops.
In Muhyo and Roji's Bureau of Supernatural Investigation, George Schuyter had quite a few of them, leading to him getting a reputation among the Magical Law Society for getting his partners killed. As a result, he acquires the sword Shinokuni and the envoy Amon and sets out to practice magical law alone, but this doesn't do much good against Vector.

Comic Books

Smax and Stochastic Fats from Top Ten. Naturally, he has a hard time warming up to his new partner, Toybox. A similar situation occurs later in the series, when Girl One dies. Her partner, Irma Geddon, considered her family and was very close with her. As such, she treats her new partner, Joe Pi, with extreme hostility for a while.
At the beginning of The Simping Detective, Jack used to have a partner named Cheyne. He and Jack killed the brother of someone very important. Jack is a tad more resourceful than Cheyne.

Film

Foxxy's partner in Austin Powers in Goldmember was killed by the eponymous villain.
Det. Sykes' previous partner in Alien Nation.
In Who Framed Roger Rabbit?, Bob Hoskins plays a private investigator who developed a sour disposition, a drinking problem and a hatred of cartoon characters after the murder of his brother.
In OSS 117 Cairo Nest of Spies , the plot is set in motion by the death of OSS 117's partner, and subsequently each mention of Jack causes OSS 117 to look wistful while Ho Yay-laden flashbacks appear on screen. Of course, He's not really dead. Since the whole film is a parody, this is Played for Laughs.
Inspector Lee's previous partner in the Rush Hour canon was murdered by the Big Bad of the second movie.
Pretty much all of Dirty Harry Callahan's partners end up dead or in the hospital, as he notes.
In The Maltese Falcon, Sam Spade notes that one has to avenge one's dead partner, regardless of one's personal feelings towards the partner.

Literature

Vincent's partner, Ernie, in Anonymous Rex is dead. Casual Rex is a prequel, leading up to how that happened.
The death of Nathan's partner in Every Demon Has Its Day is what drove Nathan to leave Texas and go home to Dogwood County.
While not involving police officers, A Christmas Carol has Ebenezer Scrooge and Jacob Marley, to whose ghost Scrooge talks at the beginning of the story.
This is part of Officer Borsch's backstory in the Sammy Keyes books.

Live Action TV

This is part of Hardison's character background in the Leverage episode "The Bank Shot Job".

Cop: I think we should go by the book on this one.
Hardison: The book?! The book got a good man killed! (turns away, apparently overcome by emotion)
Parker: Ex-partner. Probably shouldn't mention the book again. Or propellers.

Randall and Hopkirk (Deceased) and its remake both involve a cop working with the ghost of his deceased partner.
In one episode of Castle, this trope was played with like mad for Esposito's backstory.
In Tensou Sentai Goseiger: Gosei Blue's partner, Gosei Green, was killed in action before the series began, which is why he's the only team member from the Seaick tribe.
In The X-Files, Mulder has this moment when paired with rookie Alex Krycek during season two. Scully has her moment when paired with John Dogget in season eight. Subverted in the fact that the other wasn't dead, only missing.
Miami Vice: Crockett's first partner falls afoul of a car bomb early in the pilot.
Mike Logan (Chris Noth) from the early run of Law and Order had an unfortunate tendency to lose partners. One got shot and partially crippled. Another got shot and killed. The third outlasted Logan—yet still died years later.
Law and Order UK: DS Ronnie Brooks' backstory includes the fact that one of his partners was killed. What makes this worse is that it's his current partner DS Matt Devlin who accidentally reminds him of this—a year before Matt himself was killed in a drive-by shooting. However, Ronnie seems to subvert this trope, as neither of these tragedies have prevented him from forming good relationships with his subsequent colleagues—in a particularly wrenching scene, he referred to Matt as being "like my son", and it's his newest partner who seems to be rebuffing his efforts at friendship.
Adam-12 had Officer Pete Malloy, who was about to quit the force because his last partner got killed, get paired up with rookie Jim Reed. Malloy is...less than enthused by this. Later in the series, we see the reason why Malloy felt so strongly - his dead partner was a good friend that went to the Police Academy with him.
Detective Madsen in J.J. Abrams' Alcatraz is like this at the beginning of the series, though we actually see the incident in question before we see her refusing to take a new partner in the next scene.

Western Animation

Buzz Lightyear of Star Command has this, too. It's later revealed that his old partner didn't die, but defected to Zerg's side.
In COPS, Sundown's old partner Johnny Yuma didn't die... he went heel. As Bulletproof puts it, "Yuma never forgave Sundown. I think Sundown never forgave himself."
Parodied on The Critic, Jay reviews a Dirty Harry expy cop film in which the Cowboy Cop is assigned a new rookie partner fresh out of the academy, who spontaneously combusts upon his introduction.
Arcee of Transformers Prime has, as of now, lost two of her partners to Decepticons, both rather horrifically. This has left her with little patience, but great protectiveness, for her newest partner: the human Jack. It took a near death experience for her to accept him as such.

Video Games

In Mortal Kombat, Sonya Blade's backstory is that she is pursuing Kano because he killed her partner.
Ace Attorney Investigations: Jacques Portsman likes this trope so much that he invoked it personally. Poor Buddy Faith.
- Later (which is actually earlier, it's a flashback,) Detective Badd suffers this. Showing him evidence relating to his partner's murder is one of the few things that make him actually sentimental.