Fuuko is a first year student at Tomoya's school. She is always alone, making wood carvings of starfish with a small knife to give to others as presents. This hobby completely drains her concentration and awareness of her surroundings.
On the first day Fuuko went to her new school, she got hit by a car. After that she in a coma, but her spirit is wandering around at school, carving starfish. She is making the starfishsculptures to hand out as an invitation for her sister's wedding. There even is a story going around at school that there is a girl somewhere at school who got in an accident which is about her.
The first time Tomoya meets het she is carving starfish in an empty classroom. He takes her knife away from her, because she accidentally cuts herself while carving the starfish. A while after that Fuuko tells Tomoya she wants her knife back, not wanting to admit her hands hurt. Tomoya gives her her knife back. He and Nagisa help Fuuko handing the starfish out after that and they take Fuuko to Nagisa's home where she can live for a while.
Whenever Fuuko thinks of or sees something she thinks is cute, she gets into a trance. In this trance she is unaware of her surroundings and it sometimes seems as if she doesn't know she gets into a trance herself.
Tomoya, Nagisa and Nagisa's parents help Fuuko a lot with carving starfish and handing them out. Soon a lot of people know Fuuko and the starfishsculptures. But as Fuuko gets weaker in the hospital, her spiritual powers also get weaker and the people at school begin to forget her. Even Tomoya and Nagisa forget her.
But at the day of her sister's wedding everybody remembers Fuuko again and even her sister sees her for a while, she couldn't see the spirit of Fuuko that was wandering at school. After she gives her sister a starfishsculpture, the spirit of Fuuko vanishes. Everybody forgets about her the day after and Fuuko only appears a few times in Clannad after that.
Fuuko will probably wake up in Clannad After Story, because you can see her in the opening hugging Ushio.