μ appears as a young woman with white hair tied into a ponytail with violet eyes and pale skin. She wears a white dress with a low cut, possessing white wings on the back of the hips and white opera gloves. On the top of her head, she wears a small gold crown in what appears to be a rose. She also has a few gold decorations on her body, such as the ornaments circling around the back of her torso. Wrapping around her waist is a gold band, attached to a plate with a heart inscribed on it. She also wears knee-high white stockings with metal ankle guards. μ also wears what looks like a cape down her shoulder, colored blue, white and red. Two strips of cloth of the same color also fall from her waist.

μ is a very kind and upbeat individual, often performing concerts to help encourage and ease the people's worries. Cheerful would be the best way to describe her, though she can also easily be disheartened when she sees anyone who isn't quite satisfied with Mobius. When she and Aria were first creating Mobius, she often fretted that no one would enjoy it. She is also very determined, wanting to grant the wish of everyone in Mobius, though this has also strained her relationship with Aria.

Initially, μ was a simple computer program, tasked with creating new songs. However, as time went on, she and other voice synthesizer program named Aria, began to develop a consciousness and an ego. Through the songs people started to make with them, μ believed that the real world was very harsh and cruel and thus wanted to create a place where people could be happy. Alongside Aria, she created Mobius - a safehaven for people that didn't have a place in the real world. Initially, when people first started to come to Mobius, μ worried that no one was enjoying it. Overtime, however, she started to gain more influence, which led to Aria losing a great deal of power.

At a certain point in time, μ met a man named Asuka Natsume, who harbored the wish of destroying reality. As destroying the real world would also mean negating the wishes of others in Mobius, μ was hesitant to grant it. In order to be more compliant, operating under the alias of Thorn, he began to distort her view on reality through various songs. He also suggested creating a group that would be able to help her, thus founding the Ostinato Musicians.

(Source: Caligula Wiki)