In original concept dead Master is the Black Rock Shooters Sister.
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It involved two sisters and one betraying the other and turning evil.
However, they are changing this for the anime
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This plot eventually became the inspiration for the 2010 OVA, although the former sister, now called Dead Master, will be known as a friend (when in civilian form).
In either case she is the villain of black Rock Shooter. Beyond that is only speculation. Cant wait for the anime.
Not much else on her on vocaloid wiki but they will probably update once the anime is out and we know more.
http://vocaloid.wikia.com/wiki/Black★Rock_Shooter_Anime
