She was very close mates with Adam McKinnon. After he confessed his love to her, however, she said she couldn't be with him and it would be too awkward for them to be mates anymore. By the time she realized she was actually in love with him, too, he was dating Prudence Daly.
Warning: contains SPOILERS.
Marlene Price's personality rather resembles Lily Evans's; she is popular, witty, intelligent, and deeply caring. However she is not quite as selfless, romantic, reflective, or diligent in her studies as Lily.
Throughout The Life and Times, she makes huge progress as a character. At the beginning, she is meek and submissive to her abusive boyfriend, Miles Stimpson, but as the story goes on, she begins to assert herself and embrace her own wishes and dreams instead of what her boyfriend wants her to do. Cutting off her hair into a daring boy's bob is part of this transformation; it shows that she will not let other people's opinions influence her decisions anymore.
Vital, too, to her development is Chapter 33 Of Marlene, the only chapter in the story that is entirely from her POV. In it, she struggles against her ex-boyfriend Miles' attempts to intimidate her. This reaches a climax in the Gryffindor/Ravenclaw Quidditch match, where she, as Chaser, comes up against him, as Keeper. He expects her to miss, having told her beforehand, "Ten galleons you don't score one goal.", but Marlene proves him wrong, outwitting him so that he misses the Quaffle, and winning Gryffindor the match. The victory not only shows that she is more than capable as a member of the Gryffindor team, but that she has truly mastered her insecurities regarding Miles and her transformation.
Originally, Marlene aspires to be a Healer, but, like Lily and others, including Adam McKinnon and James Potter, she is greatly affected by the Peverell Hall massacre, and motivated to start fighting in order to prevent it happening again. (It just makes me want to kill Death Eaters, she tells Adam McKinnon in the wake of the disaster), leading us to believe that she may follow the Auror path instead, showing again how her character has progressed - her courage far outweighs her insecurities at this point.
One insecurity she does not master completely, however, is her insecurity regarding her weight. In one chapter, Donna remarks, "Oh, c'mon. You own a mirror. You can't actually think you're morbidly obese.", reminding us how Marlene frequently goes on unnecessary diets. Adam McKinnon also remarks on it, reassuring her that, "The railing is jealous of you."
Marlene can be selfish, as evidenced by her reaction when Adam tells her of his true feelings - in her confusion, she blames him. However, she has a deep sense of morality and honour. In Chapter 36, she is racked with guilt after kissing Adam when he still has a girlfriend, Prudence (Bloody) Daly.
After "the Life and Times"Edit
Most likely, Marlene is meant to be the canon character Marlene McKinnon, who is mentioned by Mad-Eye Moody in "Harry Potter and the Order of the Phoenix". Therefore it is likely to believe that Marlene and Adam marry before the summer of 1981, when the picture of the Order of the Phoenix, of which she was a part, was taken.
If Marlene indeed is Marlene McKinnon, which we don't have any reason to believe she isn't, she will be killed, along with her family, only two weeks after the picture of the Order of the Phoenix was taken.