Penance's origins are a mystery. Marius St. Croix (Emplate) was a vampiric sorcerer who fed on mutant energy, and claims to have fed from Penance for years. When his sister Monet refused to join forces with him, Emplate merged Monet into the Penance form, calling this her penance for rejecting him, and ordered her to accompany him across dimensions, claiming only he could restore her. Emplate chained Monet in a cell for months, feeding off her and weakening her. In the meantime, their younger twin siblings Claudette and Nicole used their own powers to merge themselves into a duplicate of Monet's form as "M." They eventually joined the Massachusetts Academy branch of Xavier's School, and when Emplate came there seeking mutants to feed from, the mutant Gateway freed Penance and sent her to the Academy. Gateway had played a role in the deaths of the Academy's previous students, the Hellions, and felt Monet's rescue to be his penance for that. He left schoolmaster Banshee with that single word of explanation, "Penance," and Banshee took this as the transformed Monet's name.
In shock, and confused by the presence of "herself" (actually the twins in her original form), Monet initially fled the Academy but soon came to trust the Generation X students. Puzzled at having seen "Monet" at the Academy, Emplate returned to Penance's cell, found her missing, and sought her out. Rebelling against Emplate's hold over her, Penance helped Generation X drive him off. Though her Penance form's muteness and Monet's natural aloofness kept her isolated, she aided the students repeatedly, notably when she shredded Black Tom Cassidy's plant form to save the school. Eventually, M reverted back into the two young St. Croix twins. When Emplate again attacked the school, the twins united once more to become M, and then they merged with Emplate to form the composite being M-Plate. Penance and Generation X hunted M-Plate and liberated the twins, thus reverting him to Emplate. Claudette and Nicole's sharing of Emplate's mind revealed Penance's true identity to them; they became M again, but then merged with Penance and instantly split into two forms: Monet assumed her original form, while Claudette and Nicole were trapped jointly within the Penance form, unable to become separate individuals again.
Monet promised to do all she could to restore her sisters, and Penance took a basement room in the academy, aiding Generation X vs. Bianca LaNeige, Paradox, Brawn and others. Penance occasionally suffered from rhapsodic fugue states that the autistic Claudette was subject to. Emplate again attacked the school (claiming to be only Emplate and no longer Marius), but both Penance and Emplate were caught in an explosion which left four unconscious bodies: Emplate, Claudette, Nicole, and Penance. They all recovered, and Penance escaped into the forest. Believing she had maimed someone, Generation X hunted her, but she helped exposed the true attacker and returned to the Academy, where she carved "home" into the bark of a tree. The school closed soon after, and where Penance went is unrevealed. Who currently inhabits Penance's form remains a mystery, but the current Penance can read and write English and has artistic talents, crafting a lifelike ice sculpture of a staff-bearing man, presumably of personal significance.
Sometime later, while bringing down a Mutant Growth Hormone drug dealing facility, the Loners' member, Ricochet used documents recovered from the facility to investigate another facility, where he happened upon a young woman bearing the resemblance of Penance, (although she answered to the name of Hollow) floating in a large glass tube.