informer

noun3

1. •
   One who informs someone else about something.
2. •
   A person who tells authorities about improper or illegal activity.

   • Being an informer often meant living in fear.
   • The police relied on an informer to catch the gang.
3. •
   One who informs, animates, or inspires.

   • Nature, informer of the poet's art. — 1729