The very first case I know of where someone who was undeniably killed in the original nevertheless shows up in a sequel is Ayesha, “She who Must be Obeyed”, from H. Rider Haggard’s She: A History of Adventure (1887). There’s no question that she has died of extreme old age at the end of the novel. Nevertheless, she came back, reincarnated, in the sequel Ayesha: The Return of She (1905).
Since then lots of characters have come back. Forgive me for mention ing those who were mentioned above.
Count Dracula – staked (discreetly off-camera) in the 1931 film, he comes back (played by John Carradine, with a moustache) in House of Frankenstein when the stake is pulled out of his skeleton (This hadn’t occurred before in a Universal movie, but it did in MGM’s Return of the Vampire, which starred Lugosi, and was clearly Dracula in everything but name. As fa as I know the concept was introduced by the movies. It had never occurred before). But he gets killed by exposure to sunlight, which reduces him to a bloodless skeleton. But then he inexplicably came back again in House of Dracula, and again gets skeletonized by sunlight.
And then, of course, he’s back, equally inexplicably, and looking like Lugosi again, in Abbott and Costello Meet Frankenstein
Speaking of Universal Horror Movies, Bela Lugosi’s Ygor was pretty clearly killed at the end of Son of Frankenstein, but he’s back in Ghost of Frankenstein without explanation. Maybe they missed.
Lawrence Talbot at The Wolfman dies at the end of The Wolfman, but they brought him back – evidently if you expose a dead wolfman to the rays of the moon it revivifies him, the way Varney the Vampyre was revived by moon rays a century earlier – in Frankenstein Meets the Wolfman. And he stayed alive through three more movies.
As I remarked in the other thread, crime boss Robert G. Durant (played by Larry Drake) was definitely killed in the original Darkman, but he was back for Darkman 2
Moriarty has been mentioned, but I don’t think it was stated that Doyle himself brought him back for The Valley of Fear, a novel whose timeline doesn’t appear to allow it to occur during the events of “The Final Problem”.
I mean, lots of others have brought Moriarty back, but they can say that Doyle himself had done it.