Ushio is Tomoya and Nagisa's daughter. She only appears in the After Story portion of the game and anime, and in the movie. After giving birth to her, Nagisa dies. This deeply saddens Tomoya to the point he gave up raising Ushio and left her to the care of Akio and Sanae. Tomoya resorts to alcohol and cigarettes to ease depression.
Five years later, Sanae arranges a trip for Tomoya and Ushio in hopes that they will be together again. During the trip, Tomoya meets his grandmother, Shino, and learns about his father's past and the sacrifices he made for Tomoya, which leads him to regret his actions, and decides to raise Ushio and acknowledge Naoyuki as his father. Shortly after Tomoya sets things right, Ushio is stricken with the same disease as Nagisa. Despite the efforts and sacrifices of Tomoya, Akio, and Sanae, Ushio's fever never subsides but worsens. In the coming winter, being filled with desperation of wanting to do anything for Ushio, Tomoya brings her for a trip, but she falls unconscious and dies shortly thereafter along with her father who succumbed to his grief. Ushio is then revealed to be the girl in the hidden world.
In the movie, after seeing Tomoya so depressed, his father asks his friends to take him to a train station where he meets with Ushio. It's hinted that he will stay with her and not end up like his father.