As her name suggests, she is presumably the paternal grandmother of Fa Mulan (the mother of Mulan's father, Fa Zhou). She is fairly elderly, with white hair. However, she appears to be spunky and isn't afraid to speak her mind. She seems to be well versed in Chinese traditions.
According to the film directors, as the (currently) oldest living member of the Fa family, she has a connection with the ancestors (despite her dismissive attitude toward them "for being dead").
Grandmother Fa first appears in town waiting on the tardy Mulan alongside Fa Li. When Fa Li laments that she "should have prayed to the Ancestors for luck" (despite the fact that in real life, Chinese women during this time were never allowed to pray to the ancestors), Grandmother points out that they can't be that lucky as they are dead. She then reveals that she has purchased a lucky cricket, Cri-Kee. She tests out the cricket by crossing a busy road with her eyes closed ignoring Fa Li's pleads and causing chaos and a massive pileup in the road.
While the women are preparing Mulan for her meeting with the Matchmaker, Grandmother provides Mulan with several items: a pendant, which supposedly helps balance, an apple for serenity, and jade beads for beauty. She then places the cage with the cricket under the upper layer of Mulan's skirt. When the matchmaker reveals herself, Grandmother quietly takes note of the woman's unpleasant disposition.
When Chi-Fu arrives with conscription notices, Grandmother goes out with the adults, slyly pointing out to Mulan, who has been told to wait inside, that she can watch from the rooftop. When Fa Zhou accepts the conscription notice, she is saddened, as she knows that Fa Zhou will likely not survive the war due to his age and injuries. When Mulan argues with Fa Zhou telling her that he will learn her place just like he did cause Mulan to run in tears, Fa Zhou just ignores her and continue eating and Grandmother was saddened at this.
That night, she awakens after the family temple reacts to Mulan's disappearance. Grandmother alerts the rest of the family and prays to the ancestors to protect her.
Grandmother Fa is not seen again for a while; her next appearance is at the end of the movie when she sarcastically comments that Mulan only brought home a sword when she should've brought home a man. She is cut off by Li Shang, who has come looking for Mulan and then she and Fa Li wordlessly point Mulan out to him. After he thanks them and walks off, Grandmother notes Shang's good looks and asks Fa Li to sign her up for the next war. When Mulan invites Shang to dinner, Grandmother asks him to stay forever.
Grandmother Fa plays a smaller role in the sequel. She predicts Shang's proposal to Mulan and even makes a bet with her son (which she lost) when the proposal happens. She would be the maternal great-grandmother of Mulan and Shang's future children.