Molly's parents were telepathic mutants who posed as a doctor and a speech therapist. Their identical power signatures and an off hand comment about being "pureblood" has led to speculation that their union was incestuous.
Molly was by far the youngest of the group, and while naïve, she was quite precocious, seeming to know more than she should for an eleven-year-old. She was the most idealistic member of the Runaways, and was the only one who made a costume. Molly thought that running away was a great adventure, but had to be continually reminded that her parents were supervillains. Out of all the Runaways, she was the only one who truly missed her family.
During a fight with Cloak and Dagger, Molly managed to rip off Cloak's cloak and revert him to Tyrone.
Molly admired fellow mutant super heroes, such as the X-Men and X-Statix, is a huge fan of Dazzler (despite constantly calling Dagger by the pop star's stage name) and had a crush on Wolverine. Originally she wanted the codename Princess Powerful, but instead Chase dubbed her Bruiser as payback for laughing at his codename.
When the Runaways were split up by the Social Services, Molly was picked up by a local branch of the X-Corporation. Three months later, Karolina organized a reunion. The kids decided to stay together rather than return to their new lives in the Leapfrog. During a confrontation with Excelsior, she revealed the Chamber impostor.
Molly maintained her crush on Wolverine until he threatened her, and she then threw him out of a window of the Church they were in. After this encounter, the X-Men tried to take Molly to Xavier's School for Gifted Youngsters for training. The two teams fought until Emma Frost decided they shouldn't force someone to come with them against their will. Later, Molly was knocked unconscious and awoke to find that she had been kidnapped by the Provost, who used children to commit thefts. Molly organized her fellow captives, defeated the Provost, and returned home.
After Gert was killed, Molly convinced the Leapfrog to say that Gert was in Heaven instead of dead. In the wake of Gert's death, Chase, who usually acted like a big brother toward Molly, became withdrawn.
Whenever the team separated to buy supplies, Molly was paired with new teammate Victor Mancha. Victor wondered if this was because Molly was the only member of the team strong enough to take him down if he unwillingly went rogue. Molly promised not to treat him like a baby, which revealed her own need to be treated as a young adult.
Molly is one of the 198 mutants who retained their abilities after M-Day. She encountered the Beast at the La Brea Tar Pits and the two discussed extinction. When Beast explained that mutantkind was heading in that direction Molly stormed off, calling him a depressing jerk. The two were able to reconcile when the Beast told her that, despite this, she should try to do as much as possible so she could be remembered.
Superhuman Strength: Molly is capable of lifting upwards of 100 tons. The upper limits of her powers have never been tested. Molly is seen toppling monsters as large as skyscrapers, tunneling through miles of rock and lift a car with ease.
Invulnerability: Molly appears to have some-degree of invulnerability when her powers are active.
Molly has demonstrated super-strength with a limit of approximately around 100 tons.
Molly's strength rapidly drains her endurance, often causing her to fall asleep after using her strength. She is now capable of using her powers at moderate levels for extended periods of time without falling asleep, but she still needs to rest afterwards and the greater the strength she uses, the more tired she becomes. Her youth may contribute to this, or it may be the result of her parents' using their telepathic abilities to force her to sleep.