Discovered buried in a garbage pile within the scrapyard of Liverchester, μ is a girl suffering from complete amnesia, with no memory of her origins or past. An input jack on her lower back identifies her as a Player, a type of individual capable of piloting combat mecha known as Equipment to defend humanity from threats called the Earless.

She has long silver hair styled into straight bangs, with two strands looping behind her ears, pale blue eyes, and thick gray eyebrows. Her typical attire includes a hot-pink-and-white short-sleeved crop top, jean shorts, wide matching boots and bracelets, a collar holding a valve from a tube amp, and two circular speakers attached to her belt.

She pilots the AC30 amplifier unit, built and maintained by Echo Rec, the boy who found her. Together, they embark on a journey to recover her lost memories and explore her potential connection to the legendary Player Jimi Stonefree, suggested by her Core part being identical to his and her similar physical appearance, including silver hair, light blue eyes, and a comparable build.

Her personality is often bullish and determined. As the journey unfolds, events from Jimi Stonefree's past—specifically a betrayal by those in power who sought to destroy the Earless rather than coexist—prove intrinsically linked to her existence. This leads to a similar betrayal where she is manipulated through the application of aerosol teen spirit, a substance that boosts a Player's power but drives them toward insanity. This causes her to lose control and become a figure of hatred for other characters, such as Nir. The journey remains focused on uncovering her origins and unraveling the mysteries of her predecessor.