Ushio is Nagisa and Tomoya's daughter, who looks more or less, exactly like Nagisa. The women have an insane dominant gene.
In the anime Ushio is also the girl in the Hidden World.
Ushio asks the Robot why he is there and he says he chose to be born there.
-Written BY Yandere
In the movie, which I absolutely adore, Nagisa dies after childbirth, giving birth to Ushio, who she named because it goes along with her name which means 'Beach'. She dies with Ushio, her family and Tomoya at her side, happy to have seen her daughter before she passed on, but because of this Tomoya goes into a massively dark depression and can't see Ushio, basically at all, it appears.
He locks himself away in their apartment and thinks about her all the time. His friends try to get to him, but they can't - no one can.
Tomoya waits constantly in the darkness, explaining that the world that he used to dream about is real now, so he can't wake up anymore from it and he just waits for it to end.
During this time Ushio is raised by Sanae and Akio, Nagisa's parents, up until the age of about five years old.
Tomoya's father comes to visit him and seeing that Tomoya has fallen into the same state he had went into, he goes to everyone in a desperate attempt to save Tomoya and Ushio's relationship.
Tomoya's loved-ones trick him into an outing, but once there, they leave him alone and have him come across Ushio, Sanae and Akio, with nowhere to go and nothing to do but face her.
Tomoya can not face Ushio, but the moment she makes a sound, he is captivated, as she looks exactly like Nagisa and is holding a little Dango toy, which she wants to share with him.
Unable to move once his daughter comes to him, she trips and almost falls, much to Tomoya's devastation and he catches her in a perfect heroic father catch.
At this point Ushio, who is on Tomoya's chest, looks up with her huge golden-brown Nagisa eyes and says, "Papa?" Meaning she totally recognizes him and knows who he is and wanted to be with him so badly and he nods a little, with tears in his eyes and replies, "Konnichiwa, Ushio," much to Ushio's delight and she replies with, "Konnichiwa." Then they both laugh and cry and hold each other and have a flash of happy Nagisa in her mother's wedding dress, happy at their tree.
This suggests that they have a happy ending in this story, which is great compared to all the others.
-Written BY Yandere
Ushio is Tomoya and Nagisa's daughter. She only appears in the After Story portion of the game, and in the movie. After Nagisa succeeded giving birth to Ushio, 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.
5 years later, Sanae arranges Tomoya and Ushio for a trip in hope for them to be together again. During the trip, Tomoya meets his grandmother, Shino and learns about his father's past that leads to him regretting his actions, and decides to raise Ushio and acknowledge Naoyuki as his father. Unfortunately, Ushio is struck with the same disease as Nagisa not long after this event. In the coming winter, wanting to do anything for Ushio, Tomoya brings Ushio for a trip, but Ushio fell unconscious and dies shortly after, and presumably Tomoya dies. Wanting to save themselves from this tragic fate, Ushio becomes the girl in the illusionary world, calling garbage doll (Tomoya) to collect the happiness around the city to create a miracle and save Nagisa and themselves.
