Hermione is a very clever girl and a skilled witch, but her penchant for being somewhat of a know-it-all doesn't endear her to many of the students, including some in her own House at Hogwarts (Gryffindor, the same as Harry and Ron). When she joins Harry and Ron to fight a troll in the first book and film, 'Harry Potter and the Sorcerer's Stone' (originally released as 'Harry Potter and the Philosopher's Stone' in the U.K., their friendship is sealed.
Hermione's character has progressed through the series as a highly intelligent witch who also helps Harry and Ron in a number of close calls, including their recovery of the Sorcerer's Stone (Year One), the clearing of Sirius Black's name as a murderer (Book Three, 'Harry Potter and the Prisoner of Azkaban') and the eventual defeat of Voldemort in Book Seven, 'Harry Potter and the Deathly Hallows'. It is with Ron and Hermione's help in the last book that Harry finds the remaining Horcruxes which hold pieces of Voldemort's soul, allowing him to be defeated.
Hermione's cleverness and skill with spells is a huge help to Harry and Ron, but the bravery and loyalty of all three characters combined is what completes their bond and makes them so successful as a trio.
Hermione and Ron often squabble, including a period in Book Three where they don't speak for a time because Ron mistakenly thought that Hermione's cat, Crookshanks, had eaten his rat, Scabbers. In Book Four, 'Harry Potter and the Goblet of Fire,' it becomes apparent that Ron and Hermione have feelings for one another, but they don't actually become a couple until the final book.
In the meantime, Ron carries on a relationship with Lavender Brown, which makes Hermione jealous and she in turn attempts to make Ron jealous as well. Hermione grows from a bushy-haired girl with large front teeth into a beauty, and she has admirers such as Viktor Krum (Book Four) and Cormac McLaggen (Book Six). In the seventh book's epilogue, readers discover that after leaving Hogwarts and helping Harry defeat Voldemort, Ron and Hermione marry and have children.
J.K. Rowling, the author of the Harry Potter books, has revealed that Hermione's character was somewhat based on herself as a young girl.
In the Harry Potter movies, Hermione's character is portrayed by Emma Watson. As with Daniel Radcliffe (who plays Harry on-screen) and Rupert Grint (Ron), she has essentially grown up on film.