Dr. Hugh Strickland first appears in the framing sequence of the novel The Revenge of Dracula by Peter Tremayne. Strickland writes to Tremayne objecting to his novel Dracula Undead, which purports to be the memoirs of one of Vlad the Impaler's sons. Strickland, like most people, thinks Dracula to be a fictional character, but assumes that Tremayne has drawn on legitimate sources, because Strickland has found an old manuscript written by an inmate of a lunatic asylum which is consistent with Tremayne's book. Since Tremayne maintains that Dracula Undead is genuine, in his view this makes Strickland's discovery a true account as well. The text is published as The Revenge of Dracula, with an afterword by Strickland which maintains his position that the story is simply the product of a deranged imagination.
Strickland reappears in the novel The Vengeance of She, in which he is presented as a friend of the historian Peter Beresford Ellis, who briefly appears as a character (Peter Beresford Ellis, of course, uses "Peter Tremayne" as a pen-name for his fiction). Over the course of the novel, tragedy strikes as the spirit of Ayesha from H. Rider Haggard's She and sequels tries to take over the body of one of Strickland's patients. Strickland is revealed to be a reincarnation of Ludwig Horace Holly.
In She, Leo Vincey is both descendent and reincarnation of an Ancient Greek called Kallikrates. He is reincarnated once more in The Vengeance of She, apparently within the same family. This may mean that Strickland is some relative of Holly - and if he is a relative on Holly's mother's side, he would be a member of the Wold Newton Family.