Informer

1. One who informs, animates, or inspires.
2. One who informs, or imparts knowledge or news.
3. (Law) One who informs a magistrate of violations of law; one who informs against another for violation of some law or penal statute.
Common informer (Law),
one who habitually gives information of the violation of penal statutes, with a view to a prosecution therefor.