Molly who is also known as "Bruiser" or "Princess Powerful", is the daughter of mutants Gene and Alice Hayes. She is the one of the youngest members of the group and has super strength mutant abilities.
At there parents annual meeting Alex Wilder, Nico Minoru, Chase Stein, and Gertrude Yorkes watch there parents murder a young women, while Karolina Dean watched over Molly. While the others met to discuss what happened at Alex's house, Molly was left at home not knowing what happened that night. The older members of the group discussed what they should do about there parents. After a failed plan of calling the cops, the kids decide to go to Gert's house to get evidence on there parents.
"Why don't you behave like the bright young woman we both know you are?" - Geoffrey Wilder to Molly
It has been hinted throughout many issues of the Runaways that Molly is actually very intelligent, and is just acting like a child to get attention.