Guan Di (關帝, Lord Guan) is the title given to General Guan Yu, who was active during the Three Kingdoms Era of China. He was deified because of his accomplishments and loyalty towards his lord. He is held in high regards in Confucianism and is also worshiped as an indigenous Chinese deity, a bodhisattva in the Chinese Buddhist tradition and as a guardian deity in Taoism. He was widely popularized by the Chinese historical novel Romance of the Three Kingdoms.
In Western culture, he is often mistaken as the Taoist god of war when in fact Guan Yu does not bless those who go in to battle, but those who take an oath of brotherhood and stand up for righteousness. He is worshiped by both the Chinese police force (for standing up for righteousness) and also by the triads (who place high emphasis on loyalty and brotherhood).
In Chinese Buddhism, he is known as Sangharama and is often paired with Skanda. He is charged with guarding the Buddhist faith and dharma.
Guan Di Sheng Gong, sometimes referred to as the Holy Emperor, is fought in front of his tomb centered in the corrupted Chinatown. The demonic invasion of Chinatown enraged him and turned him into a distorted version of himself that attacked anyone who dared to come near the tomb. After being calmed, Chen Jianmin, the leader of Chinatown, gives the protagonist an ID card to enter the US military storage hidden in the sewers.
"Originally Guan Yu of the Three Kingdoms period, he was deified and worshiped as a war god. Guan Yu fought alongside his blood brothers Liu Bei and Zhang Fei, helping construct the state of Shu until he died as a result of ploy by the two rival states, Wu and Wei. Several hundred years after his death, he appeared to Master Zhiyi at Yuquan Hill, from whom he learned to be a guardian and fend off evil spirits."