Joi appears generally as a flawlessly beautiful petite young woman. The specifics of her features, such as eye color, hair color, wardrobe, and likely some aspects of her body are variable from instance to instance. She has a variety of vocal intonations and can adopt accents. She is a holographic projection and requires some kind of projecting device to manifest to the viewer/user, whether a ceiling-mounted projector for a single room, a building-scale advertising projector, or a portable "Emanator" device that allows Joi to travel with her user.
Joi's hardware is able to extensively sense the environment it is in and translates that into data for the artificial intelligence to "experience" life along with the user. The hardware senses whether Joi is projected or not.
Joi's primary role is to communicate with other entities, human, replicant or otherwise, in a manner representing the user's mate or companion. She records data "memories" as she runs, allowing her "personality" to change over time.
Joi's program and memories reside on a main console, which for personal units is located in the home, as well as on portable Emanators or other linked devices. Joi hardware has the ability to utilize communications networks to contact other entities on its own initiative, and can be used to monitor or track Joi users.