watcher

One that watches or observes.

One who keeps vigil, as at a sick person’s bedside.

One who or that which watches.

One who keeps awake for the purpose of guarding or attending upon something or some one; a nurse, watch man, sentry, or the like.

One who observes: as, a watcher of the time.

A spy; one sent to watch an enemy.

One who watches; one who sits up or continues; a diligent observer; specifically, one who attends upon the sick during the night.

someone who watches or observes

someone who keeps vigil

a guard