Mary was one of the active crew members of the Project SEEDS ship, and on friendly terms with Rem; seen once having lunch with her, Vash, and Knives. Mary was also the object of affection of fellow crew members Steve and Rolan. Though it seemed she didn't have feelings for either, she commented to herself that it would be boring if there was only one.
Steve was generally a problem for her, with him often groping her. After one occurance in the cafeteria where Steve was told off by Knives about it, it was implied that a plan was then introduced by Knives to Mary and Rolan. Later on Steve was detained, where Rolan claimed that he saw him enter Mary's room, and where Mary claimed that he had raped her. Without a trial, he was cryogenically frozen.
Afterwards Rolan proposed to Mary, even handing her a ring. She declined, saying he must be teasing. Rolan then revealed that he had rigged the freezer to kill Steve. Mary was shocked, saying she didn't mean for them to kill him. Rolan then said that makes them partners now. Their "partnership" didn't last long though; Rem and the twins find Rolan shortly afterwards where he had shot Mary multiple times. He tells Rem that it was her own fault, that he had no choice because Mary had betrayed him after all he had done to help her. He then points the gun at Knives, saying it must be his fault that Mary had to die, though Rem jumps to protect Knives. The stand-off continues until Joey opens a hatch that sends Rolan and Mary's body into space. Interestingly, a situation almost exactly like this plays out later near the end of the series and shows that Rem's attempt to reach Rolan might have succeeded.