Warning: This article contains spoilers for Encanto.
Mirabel Madrigal has a very special relationship with Casita unlike any other character in Encanto, but this is only because of some major changes to the film. In Encanto, Mirabel is unintentionally made an outcast due to her not receiving a gift like the other members of her family, but she still finds ways to connect to her Madrigal heritage. This is shown through her bond with Casita, the magical house which provided sanctuary to Abuela, and now houses the magical Madrigals. However, Disney's original plan for the character of Casita would have made the movie very different.
At the very beginning of Encanto, Mirabel and Casita's whimsical relationship is outlined to audiences. Mirabel excitedly calls on "Drawers! Floors! Doors!" to which Casita responds by dancing in its own way, musically moving each item Mirabel calls out. Despite Mirabel not having a gift, she is still included in the magic of the family in this way, but Disney originally had other ideas.
The initial plans had Casita bullying Mirabel, which seems strange considering how synchronized they are in the final film. This plan would have been more in line with how Abuela Alma acts towards Mirabel at the beginning of the film; excluding her because she is the only Madrigal without a gift. Abuela's initial disregard of Mirabel in Encanto is already a hard watch, with the family picture scene being especially heart-wrenching, and so Casita domineering Mirabel on top of this would have been excessively harsh.
Luckily, the final film showcases the closeness of Mirabel and Casita, and the connection between the two characters demonstrates that Mirabel is still a part of the Madrigal's even though she wasn't bestowed with a magical gift. It also sets her apart from the others as the protagonist, as she is the only one that Casita really communicates with. Even Abuela more so commands things from Casita, rather than talks with it. Mirabel's bond with Casita also provides a better set-up for the ending of Encanto, by demonstrating early on that Mirabel is the heart of the family by connecting her directly with the house which harbors the family's magic.
Being the only member of her family without a gift could have made Mirabel an extremely somber and lonely character, but she takes it in her stride. Her fun connection with Casita ensures that she is never completely neglected in the family, and makes her special in a way that no other Madrigal is. In the end, Mirabel still stands out as uniquely significant in Encanto even amidst the fantastical magic.