BlackRose is the online identity of an MMORPG player in a game called The World. She is Kite's most everpresent ally, or the most reliable in terms of being "online" for story events. She appears in a number of games in the .hack franchise, including the four-part series that began with .hack//INFECTION.
It is revealed that at some point, BlackRose's brother fell into a coma while playing The World, which is why she feels so strongly about sticking with Kite and eliminating the problem. It is also suggested that she was present for this event and that she saw what happened.
Her combat class is "heavy blade," which gives her a high strength stat and makes her more suited to physical attacks and skills.