Ushio is the daughter of Nagisa and Tomoya Okazaki to who is introduced in the second season of Clannad titled Clannad~ After Story. She differs from her mother, Nagisa due to her curious personality, therefore she will go off to any random place. Later in the series she is found to have the same disease as her mother and unlike her mother, doesn't get to make it past her toddler years. She dies in episode 21 after telling Tomoya that she loves him.
an extended description!