watcher

(chiefly as the final element in compounds) Someone who observes something closely for professional reasons, such as an analyst or pundit.

(US politics) A political representative aligned with a candidate sent to observe elections to report on irregularities. A scrutineer.

Someone who keeps vigil.

A guard.

(chiefly historical, Judaism, Christianity) A kind of angel appointed to watch over the human realm, in particular one of those who became fallen angels and begot the Nephilim.