The term WATCHFUL ONE can be described by three different crossword solutions: SENTRY (6 letters), EYER (4 letters), and GUARD (5 letters). A SENTRY is a soldier or GUARD assigned to stand and keep watch, usually at a military post. An EYER is someone who watches or observes carefully. A GUARD is someone who is responsible for protecting and defending a person, place, or thing. All three of these terms describe someone who is watchful and vigilant.