As a little girl, she is assertive and knows karate. As the story progresses, she is shown in the future composing her own music and teaching frogs to dance and sing in a big band and jazz musical style. She is a caring, charming, fun-loving and inviting mother and wife. She is extremely maternal and compassionate, shown when she offers to take Lewis in knowing he had no family, unsuspecting that Lewis is "Cornelius", her husband, from the past. As stated by her and her husband, she is "always right", even when she's wrong.
Franny is first introduced at the Inventco Science Fair as the "Annoying Little Girl" that tells Wilbur not to sass her. Wilbur has no idea she is his future mother. He also knocks her frogs out of her hands and has to help her pick them up until he sees what he thinks is the Bowler Hat Guy and runs off. She is later more properly introduced in the year 2037 at the age of 40, a wife and a mother who successfully teaches frogs to sing and play jazz music. While Lewis is searching for the garage, Bud takes him to her music room and she makes him join her band on maracas. She later calls for the boys to come to dinner and threatens to come get them if they don't come, which they have to comply with because otherwise, she'll see the busted time machine. At dinner, she and her older brother Gaston get into the traditional family food fight due to Wilbur. Later, after Lewis's failed attempt to fix the PB & J machine, the entire Robinson family congratulates him on his brilliant failure. Franny proposes a toast to Lewis, and he says that if he ever has a family, he wants it to be like the Robinsons. When Franny asks Wilbur what Lewis meant by that, she is shocked and saddened to find out the boy is an orphan.
When the T-Rex attacks, Franny tells Lewis to run and she goes to help her brother Gaston, but they both end up getting thrown out of the battle, but Lewis and Wilbur eventually defeat it. Afterwards, after Lewis accidentally calls her "Mom" while reminiscing on his and Wilbur's victory over the dinosaur, he notices a bruise on the forehead of Franny, due to the battle, which she waves off, and she offers to adopt Lewis, and he accepts, until Wilbur knocks off his baseball cap, revealing his spiky hair. Franny is shocked to see an unmistakable resemblance between Lewis and her husband, realizing who Lewis really is. Saddened, she is forced to tell Lewis he has to go back to his own time, making him terribly upset and causing Wilbur to admit that one of the time machines was stolen (which explains the dinosaur) and that the other is broken. Furious, Franny decides to call Cornelius, but when Lewis defends Wilbur and asks to at least go back and see his mom as Wilbur "promised", Franny is furious, confronting her son and causing him to admit the truth, causing a hurt and betrayed Lewis to run off. Franny, before heading into the house, tells Wilbur that he's grounded until he dies.
When Doris and the evil Bowler Hat Guy, who is really Michael Yagoobian when he was a kid, changes reality, Franny and the rest of the family are controlled as Doris's slaves and chase him. Later, when Lewis sets everything right, Franny is concerned about him, asking if he is hurt or if he has any broken bones. Lewis tells her that he's fine and feels a lot better than he has in a long time. When Cornelius, who is in reality future Lewis, arrives home from his business trip, he is shocked to see his younger self there, and he turns to Franny questionably. At first, it seems like she doesn't have an answer, but she catches Wilbur when he tries to get away, indicating that he has everything to do with it. Cornelius understands completely, while an upset Wilbur says "ratted out by the old lady. Harsh".
Before Lewis leaves to go back to his own timeline, Franny gives him some advice for the future: she is always right, even when she's wrong. Cornelius reassures Lewis this is true, and when Lewis arrives back at the science fair, he meets a young Franny. She tells him that she believes that frogs have more musical abilities than people, but that people think she's crazy and demands to know if he thinks she's crazy too, as she takes on an aggressive stance. However Lewis, remembering what he saw in the future and future Franny's advice to him before he left, tells her that he thinks she's right, endearing her to him.
She is last seen smiling with Bud and Lucille as Lewis reflects on what a bright future he will have.
Starting before the story of the film, Franny runs into Wilbur after his adventure in Egypt to tell him to take out the trash, which is the start of his adventure. In the altered timeline, Franny laments how depressed Cornelius is working for Stanley at Magma Industries and when Wilbur fixes the timeline, she reveals that she and Lizzy have been friends since childhood.
- Her maiden name, Frammagucci, was mentioned in early script drafts before being removed.
- In the movie, while Lewis is visiting the future, Grandpa Bud introduces him to Franny. He says "I think you'll like her." This hints Lewis' connection to her before it is revealed that they are married.
- Franny and everybody else on her side of the family all have "cowlicks."
- According to the DVD commentary of the film, Franny's body model is used for Lewis's mother.
- Franny's hair as a child is similar to Lizzy's.