Brightness Shi'a is one of the two spirits taking turns to possess the members of the Dishonoured. Calling her "one" spirit is maybe overstating matters though, as this distinct side of her personality is vastly different from the one the Dishonoured got to know when they were hanging in the cell. Tenago the Not Necromancer has also alluded to Shi'a being one of several personalities in the spirit. How several different personalities are stored in one spirit is still unknown.
Brightness Shi'a has that haughtiness of a noble woman used to being obeyed with no questions. She has displayed a knowledge of politics, family-lines, and made allusions to luxury goods on several occasions. Unfortunately, her knowledge so far seems to be outdated with at least a century or two.