IPA: /ˈwɒtʃə/
KK: /ˈwɑːtʃər/
A person who observes or watches something, often paying close attention to it.
The watcher stood quietly in the shadows, keeping an eye on the event.
Watcher → It is formed from "watch" (from Old English *wæccan*, meaning to be awake or to guard) and "-er" (meaning a person who). A watcher is a person who is awake or guarding, observing something attentively.
Think of someone who is 'awake' and 'guarding' — that's why a watcher is a person who observes closely.