Ginny is the youngest Weasley child. She's fourteen now – a year below Harry, Ron, and Hermione. In her first year at Hogwarts in Book 2, her crush on Harry was played for laughs. But now, it's Ron who seems most intent on setting Ginny up with Harry. Ginny, meanwhile, is growing up. She surprises Ron utterly by telling him that she's starting dating Michael Corner, a Ravenclaw Quidditch player. When Michael Corner gets annoyed at Gryffindor's final triumph over Ravenclaw in the Quidditch Cup, Ginny dumps him and moves on to Dean Thomas, Ron's Gryffindor dorm mate.
And it's not only in romantic terms that Ginny seems to be growing up. She has become closer to the twins, Fred and George, who have given Ginny a certain disregard for the rules. So, Ginny believes that you can achieve just about anything as long as you are willing to be creative. When Harry seems depressed and miserable after watching Professor Snape's memory of his father and Sirius (see "Characters: Professor Severus Snape"), Ginny confronts Harry about why. When Harry answers that what he most wants in the world is to talk to Sirius (in spite of Professor Umbridge's spying), Ginny gets Fred and George to set up a diversion to give Harry the time to do just that.
Furthermore, when Harry overhears Moody's opinion that Voldemort may be possessing Harry (22.223), Ginny won't stand for Harry withdrawing from his friends. Instead, she points out, "Well, [hiding] was a bit stupid of you [...] seeing as you don't know anyone but me who's been possessed by You-Know-Who, and I can tell you how it feels" (23.66).
Things have gotten so rough for Harry that he sometimes forgets that other people have also had to stand up to Voldemort. Ginny is around to remind him to be less self-centered. When Ginny tells Harry how it felt to be possessed by Tom Riddle's diary in Book 2, he calms down. Harry realizes that all of his morbid worries about Voldemort controlling him like a puppet are overblown.
Ginny also insists on accompanying Harry, Ron, and Hermione to the Department of Mysteries, where she breaks her ankle but continues to fight the Death Eaters. Even though Ginny has a fairly low-key and minor role in Book 5, her character is clearly developing to become more important to Harry over the course of the series.