She is one of Lily's best friends in the same year and house as her, in fact she is more of a sister to Lily than Petunia ever was.
Her patronus is a b*tch (a female dog).
Marlene excels in Charms, Divination, and later on in Quidditch where she joins the Gryffindor team in her 5th year as James's fellow chaser.
Due to being good friends with both the Marauders and Lily, Marlene despises Severus Snape just as much as the Marauders do.
Marlene is the Fleur Delacour of Hogwarts in terms of her good looks, but without the Veela powers.
Marlene has a younger brother named Chester who is 3 years below her
Although she has a lot of boys fawning over her (except James who views her as a sister, Remus because he doesn't want to pass on his condition to anyone, and Peter because he is way out of Marlene's league), the one boy who Marlene really wants is Sirius (I ship Blackinnon), but Sirius (according to Rowling) is a player who is not really interested in relationships and also the fact that Marlene's overprotective father doesn't really trust Sirius (due to Sirius being part of the Black Family and also due to Sirius's reputation as a rebellious bad boy player).
Just as Sirius is finally willing to start a relationship with Marlene and just as Marlene is about to be made godmother to Harry, she is massacred along with her entire family (the McKinnon's were wiped out shortly after Harry was born), with Marlene and her brother Chester (due to their bravery and determination) being the last two to die in their family.