Answer: Briseis
Briseis was Paris and Hector's cousin, and had become a servant of the Sun God, Apollo. She had been found in the temple of Apollo when the Mermidans attacked and killed the priests. She was given to Achilles to "amuse" him, but then was taken by Menelaus because Menelaus was angry with Achilles for charging the beach of Troy alone, without the rest of the army. Later she was returned to Achilles. Right before Achilles died, he told Briseis that she had given him peace in a lifetime of war. Briseis played a huge part in the plot, she almost cost the Greeks the war by "distracting" Achilles with love.
The legend says that Briseis was not taken from Achilles because Agamemnon was mad at him. He had to give back Chryseis, his prize of war, to make the gods happy and he wanted Briseis to replace her.