WARNING: This contains spoilers for Fullmetal Alchemist: Brotherhood.Fullmetal Alchemist: Brotherhood introduces numerous prominent characters that contribute significantly to the storyline. But none of them compare to Winry Rockbell, Ed and Al's childhood best friend, and, ultimately, Ed's future wife.
At first glance, Winry's significance in the series may not seem all that obvious, seeing as she takes up a supporting role. However, as the show progresses, it becomes clear that Winry may be one of the most important characters.
Her Intelligence
The series introduces Winry as an exceptionally bright young woman with a deep passion for the art of mechanics. Taking after her grandmother, Pinako, Winry desires to pursue a career as an automail engineer. Following the transmutation incident that causes Ed to lose his limbs (and Al to lose his body), Winry's passion for mechanics and love for Ed drives her to produce automail prosthetics as replacements for his lost arm and leg. From then on, Winry becomes his personal engineer and medic. This is an especially important element in the series because, without Winry, Ed wouldn't have obtained prosthetics so early in his life (and specifically designed for him in mind).
On the other hand, it is arguable that perhaps Pinako could have provided Edward with prosthetics because of her own ability as an automail technician and surgeon. However, Winry's talent and expertise eventually propel her to exceed her grandmother's skills, suggesting she harbors her own distinguished craft and skill. Her capability and passion for automail prosthetics motivate her to help many people in the future.
Her Friendship and Support
Another prominent piece of Winry's significance is her friendship. After the Elric boys lose their mother, then their home, they are taken in by Pinako Rockbell, Winry's grandmother. Although Pinako acts as their guardian, Winry's friendship and emotional support are potent from the very beginning. Winry's friendship with the Elric boys is quite sincere from the very beginning of their introduction. In fact, the series reveals that when the three were just children, the boys argued about who would be the one to get to marry her one day. Winry never took it too seriously and rejected both of them.
Winry's kindness has gone beyond that of merely helping the Elric boys. She is not only supportive, but she also has the ability to empathize with others who are hurting, which has frequently caused her sorrow and grief. However, similar to Ed, she can be quite headstrong and stubborn and thus, never fails to stand up for her beliefs. As a result, this has led to numerous arguments between both Ed and herself.
Despite her intelligence, Winry's role is primarily to ground the boys and act as their foundation, so to speak. Her presence clearly takes up a significant part (if not all) of Ed ad Al's motivation to complete their quest of recovering Al's body and return home. If not for Winry, it's possible the boys wouldn't have cared to return home all that much, or perhaps, even had a home in the first place. Ultimately, her role as the Elric boys' emotional support is essential to their growth, and without Winry, they most likely wouldn't feel anchored to some kind of family.
Her Normalcy
In addition, Winry is one of the few 'ordinary' characters in Fullmetal Alchemist: Brotherhood, unlike most of the characters that are tied up to the political issues of the overall plot. Winry's normalcy is an essential role in many anime shows, specifically those in which the majority of the characters are extraordinary fighters, public figures, and villains. In other words, among the chaos, death, and immense grief, Winry's presence and normalcy indicate a form of stability and grounding for the Elric brothers. She is a beacon of hope and light in an otherwise dim world.
Winry is also a critical character because she functions as a representation of the ordinary citizens of Amestris and Ishval because, like them, she bears no direct relevance to the Ishval war (or matters within the Amestris government). Nevertheless, much like many innocent families, such political affairs have affected her life by generating the death of her parents. Initially, their deaths don't appear to carry much weight within the series. However, upon further examination, Winry's parents do, in fact, become a crucial detail in the series, particularly because they link a relatively 'minor' character (i.e. Winry) to the grand scheme of things. However, despite this indirect connection, Winry doesn't necessarily involve herself in the conflict, for the most part. Instead, her connection further encourages the Elric brothers to not only complete their quest but also protect her from dangerous situations.