Heiyue is a criminal organisation with roots in Calvard and to the east, particularly with the Eastern Quarter. According to Kaela, they were originally created to help eastern immigrants as a mutual aid society. They are known to operate across the centre of the continent and have branches in at least Calvard, Crossbell and Ored States. Arios MacLaine states that they are wholly responsible for controlling Calvard's underworld, although divided in several different units.
They are led by a council of elders. Agents of Heiyue are typically trained in hand-to-hand martial arts and wear eastern styled black or dark coloured suits. Cao Lee, who wears a bright lavender suit, may be an exception.
At an unspecified point in the past, Heiyue and the jaeger corps Red Constellation came into conflict in the Eastern Quarter. Whilst the exact circumstances of the conflict are yet to be determined, it is known the Red Constellation withdrew and Heiyue are considered the 'conquerors of the Eastern Quarter'. Cao states that a deal was made, and there is no grudge, but that the elders still get angered at hearing their name.
One branch was established in Crossbell to take over the black market from Revache. Cao Lee, representative of the Crossbellan branch of Heiyue, disguised Heiyue's building as Heiyue Trading, Ltd. in the Harbor District. The SSS investigate the trading company in hopes of finding a connection to the mysterious assassin, Yin.
Heiyue's appearance in the city soon puts Revache in an increasingly defensive position as they lose ground to the syndicate. The agents of Heiyue and their subcontracted assassin Yin, the 'Demon of the Eastern Quarter' are more than a match for those of Revache. Pushed to ever-increasing desperation, the mafia turn to a drug, Gnosis to increase their abilities, allowing them to raid Heiyue's office and overpower them. The sudden strength of their adversaries is enough to temporarily put a stop to Heiyue's operations in the city, but not for long.
With Revache all but dismantled, Heiyue are swift to fill the vacuum of power, quickly acquiring their former rivals' assets and properties, with the exception of the old headquarters which is instead bought by the Red Constellation who have arrived in the state. The grandson of one of the elders, Xin is also dispatched and placed under Cao's care. Cao enlists the SSS to help introduce Xin to the city and he takes a liking to them.
Heiyue are hired by President Samuel Rocksmith at the West Zemuria Trade Conference to provide additional security, and they make short work of the anti-immigration terrorists that attack the conference. They are also stated as to have been involved in the arrest of Republican terrorists in Altair.
The Red Constellation later raze the city, blowing up the rebuilt Heiyue headquarters. Heiyue retain their network of information and pass on information about resistance efforts mounting in Mainz. Heiyue become opponents of the new independent Crossbell and their agents help in the eventual plan to retake the city.
Cao relieves Yin of her contract with Heiyue, but states that they will need her power for their future plans.
Cao makes a brief appearance helping Lloyd and Rixia in the Geofront via a monitor on a terminal. Heiyue appears to be supporting the resistance efforts against the Empire at this time.
Heiyue have relocated their base of operations in Calvard. They are working behind the scenes to deliver information and supplies to the SSS and their allies trapped by Operation Birdcage; new Class VII are co-opted into helping them do this, unaware of the true purpose.
After the Great Twilight, Cao re-evaluates his position. Cao later makes a deal with Ouroboros and Governor General Rufus Albarea to hold back potential obstacles from stopping him access the trial for the Auric knight El-Prado. Heiyue engage new Class VII in a fight in the Geofront but Estelle Bright and Elie MacDowell come to their aid and distract them for a short while.
Cao continues to keep his allegiances fluid throughout the rest of the conflict. Heiyue and the CID form a partnership by the end of the game, with Heiyue providing assistance in exchange for intelligence.
Heiyue, under Cao's direction, appear to join forces with Supreme Leader Rufus Albarea when he interrupts the re-independence ceremony and stages a coup. Agents of Heiyue are active in trying to hunt down Lloyd and company, eventually facing him at the Sun Fort. they are defeated and retreats.
Later, it is revealed their alliance with the Supreme Leader was a ruse. Cao, knowing that the resistance would prevail against the Supreme Leader, changes sides once more and begins assisting the resistance in throwing out the Black Guard.
Heiyue have been trying to recruit Aaron Wei, but he has no apparent interest in them.