Black★Gold Saw was originally created draw as part of the Black★Rock Shooter series of art images. Her image has evolved somewhat over her different media appearances.
Here called Golden Sensei, she is a kindergartner teacher that watched over Stella, the shopkeepers granddaughter, and Su-san (Strength). She is initially skittish at meeting De-chan and Rock-chan do to unspecified "dangerous things recently." She warms up quickly to the two, expressing an interest in playing dress up with De-chan and later the teenage Rock-chan.
Black★Rock Shooter OVA:
Black★Gold Saw only appears in the beginning, doing battle with Black★Rock Shooter. Her reasoning or purpose are not explained. She is shown to be an expert swordsman and exhibits great strength, speed and agility during the fight. Black★Gold Saw cuts into Black★Rock Shooter's side before fleeing, leaving the other to be buried under falling rubble. Her weapon is a massive sword with a serrated back edge called King Saw.
Black★Rock Shooter (2012)
Black★Gold Saw Is the other self of Saya Irino. She is seen throughout the series watching Black★Rock Shooter's activities through large red eyes in the sky. She initially tries to kill Black★Rock Shooter with a giant version of King Saw erupting from the ground. Later she is shown watching Chariot's battle with Black★Rock Shooter. Later she stops the destruction of Chariot's world by stabbing King Saw into the rip before escaping away with Dead Master. In the real world Saya is a school councilor who uses her knowledge and position to push Yomi (Dead Master's other self) deeper into a depressive state.
In the other world Black★Gold Saw sprinkles water on the ground and hooded forms grow from it. Before she can do anything else Black★Rock Shooter breaks into the world and blasts the hooded girls not killed by falling rocks. Black★Gold Saw flees the destruction of her area, while in the real world Saya sense the death of an other self related to a girl she has been grooming. Black★Gold Saw watched the battle between Dead Master and Black★Rock Shooter from a distance before disappearing yet again.
Black★Gold Saw witnesses the transformation of Insane Black★Rock Shooter and launches a desperate but futile attack. Saya merges with Black★Gold Saw to try and reach Mato trapped inside Black★Rock Shooter, even showing her backstory with Yuu/Strength but to no avail, and Black★Gold Saw is knocked unconscious, only to be saved by Strength. In the finale she appears in the distance with a group of hooded other selves, presumably under her future guidance. Her other self Saya presumably resumes normal councilor duties.