Troy

The opening scene shows the troops of King Agamemnon of Mycenae ready to fight against the troops of Triopas of Thessaly, a battle only avoided when the great warrior Achilles defeats Thessaly's champion in single combat. Meanwhile, Prince Hector of Troy and his younger brother Paris negotiate a peace treaty with Menelaus, King of Sparta. Paris, however, is having a secret love affair with Menelaus' wife, Queen Helen, and smuggles her aboard their homebound vessel, much to Hector's fury. Upon learning of this, Menelaus meets with Agamemnon, his elder brother, and asks his help in taking Troy. Agamemnon, who has wanted to conquer Troy for a long time, agrees, since it will give him control of the Aegean Sea. On King Nestor's advice, Agamemnon has Odysseus, King of Ithaca, persuade Achilles to join them. Achilles, who strongly dislikes Agamemnon, initially refuses, but eventually decides to go after his mother, Thetis, tells him that though he will die, he will be forever remembered.
In Troy, King Priam is dismayed when Hector and Paris bring Helen, but welcomes her as a guest and decides against sending her home, since Paris will likely follow her and be killed, choosing instead to meet the Greeks in open battle. The Greeks arrive shortly after and take the Trojan beach, mostly thanks to Achilles and his Myrmidons, among them his cousin Patroclus, who sack the temple of Apollo but allow Hector and the surviving Trojans to return to the city. Achilles claims Briseis, a priestess and the cousin of Paris and Hector, as a war trophy, but is angered when Agamemnon spitefully takes her from him and decides that he will not aid Agamemnon when they lay siege to Troy.
The Trojan and Greek armies meet outside the walls of Troy. During a parley, Paris offers to duel Menelaus personally for Helen's hand in exchange for the city being spared. Agamemnon, intending to take the city regardless of the outcome, accepts. Menelaus wounds Paris and almost kills him, but is himself killed by Hector. In the ensuing battle, most of Agamemnon's forces fall to Troy's archers and Hector kills Ajax. On Odysseus' insistence, Agamemnon gives the order to fall back. In order to keep their spirits up, he gives Briseis to the Greek soldiers for their amusement. When she is threatened with rape, she is saved by Achilles. The two fall in love, and Achilles decides that the war is a lost cause, resolving to leave Troy in the morning.
Despite Hector's advice otherwise, Priam instructs him to retake the Trojan beach in the night and force the Greeks home. The attack brings the Greeks together and the Myrmidons enter the battle. Hector personally duels a man he believes to be Achilles and cuts his throat, only to discover it was actually Patroclus. Devastated, the armies agree to stop fighting for the day. Achilles is informed of his cousin's death and vows revenge. Knowing of the coming retribution, Hector leads his wife, Andromache, to a secret tunnel beneath Troy and instructs her to take their child and any survivors she can out of the city should he die and the city fall.
The next day, Achilles arrives outside Troy and demands Hector come out. The two fight evenly for a while until Achilles wears Hector down and kills him, dragging his corpse back to the Trojan beach, straining his relationship with Briseis. Priam, in disguise, sneaks into the camp and meets with Achilles, imploring him to let him take Hector's body back to Troy for a proper funeral. Ashamed of his actions, Achilles agrees and allows Briseis to return to Troy with Priam, promising a truce of twelve days so that Hector's funeral rites may be held in peace. He also tells his men to return home without him, for he has a battle he must fight on his own.
Agamemnon becomes infuriated at Achilles' actions and goes into a crazed rant that he will take Troy no matter what. Concerned that Agamemnon may lead them to destruction, Odysseus concocts a plan to get inside the city by having the Greeks build a gigantic wooden horse from their boat parts and abandon the Trojan beach, hiding their ships in a nearby cove to make it seem as if they have left. Priam orders the horse brought inside the city as a gift from the Gods, over Paris' objections. A Trojan scout finds the hidden ships in the cove but is killed by the Greek archers before he can alert the city. That night, Greeks hiding inside the horse emerge and open the city gates for the Greek army, commencing the Sack of Troy. While Andromache and Helen are getting the Trojans to safety through the tunnel, Paris gives the Sword of Troy to Aeneas, instructing him to protect the Trojans and find them a new home. Glaucus is killed by Odysseus. Agamemnon kills Priam, and then Agamemnon finds Briseis and taunts her, and she kills him. Achilles fights his way through the city and finds Briseis, but is shot through the heel by Paris, which makes him vulnerable. Paris puts several more arrows into Achilles' chest until he finally collapses. With his dying breaths, Achilles implores Briseis to leave the city with Paris. They escape Troy before the Greeks find Achilles' body. In the aftermath, with Troy finally taken, funerals are held for the slain and Odysseus personally cremates Achilles as the surviving Trojans head to Mount Ida.

Question: Who is Patroclus's cousin?
Answer: Achilles.