Watcher

In the television series Buffy the Vampire Slayer, a Watcher is a member of a secret organisation devoted to tracking and combatting malevolent supernatural entities (and particularly vampires), primarily by locating and training individuals with the talents required to fight such beings and win.

Notable Watchers include Rupert Giles (Buffy's Watcher for most of the series) and Wesley Wyndham-Price.

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In the Highlander television series, a Watcher is a member of a secret organisation devoted to tracking Immortals.

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Warning: Wikipedia contains spoilers

In the "Logopolis" serial (1981) of the television series Doctor Who, the Watcher was a spectral figure that would appear in the distance, watching, whenever something significant happened. He turned out to be a foreshadowing of the Doctor's next (fifth) incarnation, and merged with the dying Doctor at the end of the last episode. His exact nature has been debated by fans, and it has been speculated that certain other shadowy presences that have appeared in the series may also have been watchers.

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The Watcher is a mystery novel by Kay Nolte Smith[?].