watcher

Short definition from Wiktionary

Noun

1. 1
   Someone who watches or observes: Someone who observes something closely for professional reasons, such as an analyst or pundit.
2. 2
   Someone who watches or observes: A political representative aligned with a candidate sent to observe elections to report on irregularities. A scrutineer.
3. 3
   Someone who keeps vigil.
4. 4
   A guard.
5. 5
   A kind of angel appointed to watch over the human realm, in particular one of those who became fallen angels and begot the Nephilim.