Yuuko is a mysterious girl who Hiro first meets in a church on Christmas night. She always appears generally out of no where, and disappears just as quickly in various places throughout the story to talk with Hiro or other characters and give them advice; interestingly, Miyako already knows her. She could be considered the key character throughout the ef story.
She is a main character for the second series, part of which is focused around her connection to Himura.
Yuuko is a kind hearted person who hates being alone and puts on a brave face, even when upset or hurt.
Yuuko spends her early life in an orphanage and it is there she meets Himura. She breaks her short friendship with him after he tells her that she can't be his 'sister'. This rejection later causes her to start self-harm.
She is soon after adopted by Akira, who also lost his younger sister during the earthquake. However he is physically and sexually abusive towards her, because she resembles Akira's dead sister. Yuuko keeps this quiet though, as she has no other person or home to return to.
Many years later, while still living with Akira, she joins the same school that Himura attends by coincidence, as Akira is an art teacher there. She decides to follow Himura around to upset and hurt him, for the rejection he gave her so many years ago.
Though the feelings of anger subside and she comes to terms with the fact that many years ago, she never wanted to be his 'sister', she wanted to tell him she loved him.
After making up with Himura, and his realisation that he loves her, her life is unfortunately cut short. She is knocked down by a car while trying to save the young Mizuki as they were waiting for Himura on Christmas day, and dies alone shortly after she goes to pray in a church for the safety of her, Himura, and their child (she was carrying a child). (In the anime, young Mizuki was not the cause of her death. Instead, she was trying to pick up a small ball.)
Her spirit however continues to visit the troubled people of the EF world, unknown to Himura.
At the end of the second series Himura learns of her appearances and finds her spirit in the Japanese Otowa Church on Christmas Day. She spends one final day with him, but after correcting him, saying that he "loved her", rather than "loves her", she tells Himura her work is finally done and that its time for him to move on.
Yuuko was 16 years old as her headstone stated she had been born in 1982 and then had passed away in 1998. Thus, the time period in EF - A Tale of Melodies was 2008.