a guard who keeps watch
"The night watcher stood by the gate, keeping a sharp eye on anyone approaching after dark."
a person who keeps a devotional vigil by a sick bed or by a dead body
"After his grandfather passed, his daughter stayed by the bedside as his devoted watcher until the funeral arrangements were made."
Someone who watches or observes.
"The security camera caught a silent watcher standing in the shadows behind the building."
Someone who observes something closely for professional reasons, such as an analyst or pundit.
"The financial watcher predicted a market crash before any of the major banks had adjusted their forecasts."