watcher

noun6

1. •
   Someone who watches or observes.
2. •
   Someone who observes something closely for professional reasons, such as an analyst or pundit.
3. •
   (US) A political representative aligned with a candidate sent to observe elections to report on irregularities. A scrutineer.
4. •
   Someone who keeps vigil.
5. •
   A guard.
6. •
   (Judaism) A kind of angel appointed to watch over the human realm, in particular one of those who became fallen angels and begot the Nephilim.