SpoilerThe Messier code BK-201 belongs to her star. Bai's abilities are said to be vast, with generating abnormal electricity as only the tip of the iceberg. In addition to this, she also has the power of molecular reconstruction. Her remuneration is to sleep.
At the time of the Heaven's War in South America, Hei was constantly together with Pai during their many operations, acting as a bodyguard of sorts. It was for Pai's sake that Hei originally joined the Syndicate. When the Heaven's Gate disappeared, Pai's power became Hei's, because—according to Amber—she continues to exist within him. Hei himself remains oblivious to the fact, until the very end. Her real name is revealed at the end: Xing (which may in this instance mean "star".