sycophant

a person who tries to please someone in order to gain a personal advantage

An informer; a talebearer.

Greek sýkophантнs, lit. informer, slanderer

“I should best like to see these flung at her brother, the sycophant." "Do you know them?" asked Diodoros, hoarsely. "No," replied the old woman. "No need for that.”

— Georg Ebers, The Historical Romances of Georg Ebers

“To some psychologists, Wormtongue serves as an archetypal sycophant.”