Marlo first appeared as a Las Vegas aerobics and swimming instructor and was a love interest for the casino security enforcer "Mr. Fixit", in reality the Hulk, believed dead, and now living incognito, his true identity unknown to Marlo. She ended their relationship after seeing him brutally kill an enemy. They later reconciled after Hulk reached out to her in a rare display of emotion and their relationship was able to end in a more amicable way.
She later appeared as the girlfriend of Rick Jones, the Hulk's perennial sidekick, and best friend to his alter ego, Bruce Banner. She met Rick while he was on a book tour promoting his memoir, Sidekick. It was only when Rick, not knowing of Marlo's previous relationship with the Hulk, introduced her to Bruce and his wife Betty, that Marlo discovered that Mr. Fixit was really the Hulk, and that Bruce was not merely his friend, but his alter ego.
Shortly after this introduction, the psychiatrist Doc Samson managed to integrate the various personalities of the Hulk, resulting in a being that now possessed a permanent Hulk body with Bruce's face. When Betty expressed skepticism of a continued relationship with this new version of her husband, she moved in with Marlo, with whom she became best friends.
Marlo was stabbed to death by Jackie Shorr, Rick's deranged biological mother who had abandoned him at a young age. Rick eventually employed the technology of the Hulk villain known as Leader, who managed to revive Marlo.
Rick and Marlo eventually married. Every supervillain from Kree to Mephisto appeared at her wedding, due to the machinations of the Impossible Man. The two became even became famous while hosting a successful talk show called Keeping up with the Joneses. During this time they would drift in and out of the Hulk series, making numerous appearances. During this time Marlo and Betty Banner (wife of Bruce Banner) became close friends and even lived together for a while until Betty's untimely death.
Marlo and Rick separated for a time, and she occupied herself with running the couple's Los Angeles-area comic book store. She and Rick reunited shortly after he was bonded with Genis-Vell the son of the original Captain Marvel, who took on that mantle himself. Soon after, Marlo was menaced by the Wendigo, a murderous psychopath. She was saved by the Hulk.
The comic store itself often had stories featuring real-life incidents happening to Marlo and Rick. Marlo spent much of the time annoyed by a chain-smoking, female ghost only she could see and hear. This was the spirit of the girl killed during the Wendigo debacle. Marlo played a major role in the Captain Marvel series. At one point she developed an attraction for the superheroine Moondragon, and Rick left her. The couple later reunited after Marlo realized that whatever feelings she had, they were stronger towards Rick. As the Captain Marvel series ended, Rick, now separated from Captain Marvel, and Marlo were last seen happily in love and giving their marriage another shot.