Hatsune Otonashi (音無 初音, Otonashi Hatsune) is a minor character in Angel Beats. She was Otonashi's younger sister.
When she was young, Hatsune was diagnosed with an unknown medical condition and was bedridden for at least two years. Within this period, Yuzuru (her older brother) acted as her guardian. The absence of parents suggests that they were orphans.
Yuzuru visited Hatsune everyday, in order to keep her company and give her a manga that he bought for her with his meagre wages. Compared to her brother, Hatsune enjoys studying and couldn't wait to start school. However, her sickness kept her from going outside.
She is also gifted by her brother's love, who despite having a meagre income from part-time jobs, also depended upon her love to keep on living. Even though Yuzuru bought the manga randomly (acknowledging that he perhaps even the same issues) Hatsune was always grateful and these gestures of gratitude, in turn, reassured him.
Gradually, the effects of Hatsune's condition took its toll upon her, and her condition worsened. Despite this, she was still very keen to go outside see the Christmas decorations, a fact that she mentioned to Yuzuru who decided to do one nice thing for. Working hard to save up enough money from his part-time jobs, he planned to take her out for Christmas.
When she was refused permission to leave, Yuzuru snuck her out carrying her on his back.
The pair made it out of the hospital to the high street, admiring the Christmas decorations. As Yuzuru promised to take her anywhere and buy anything she wanted, Hatsune thanked her brother one last time before passing away with her arms still wrapped around him.
Hatsune's death had a huge impact on Otonashi, who immediately fell into a deep depression, recognising too late that it was Hatsune's gratitude and love, which kept him going in life. Eventually, however, Yuzuru snapped out of this, and began working hard to better himself so that he could become a doctor, and help others the way Hatsune had helped him. Unfortunately, he never fulfilled these ambitions.
Just before he died, Otonashi remembered his sister wishing that she had a donor, which could have made her treatment easier, perhaps even saved her. With his last ounce of strength signed a donation card, voluntarily offering his organs to people who needed them. He died, shortly afterwards, but not before inspiring the dying people around him to sign their own donation cards.
Upon entering the afterlife, Yuzuru originally had no recollection about who he was nor about Hatsune's existence. He was devastated when he found out what had happened and wept openly for her. 
- Her first name, "Hatsune", means "first sound". Ironically, her surname, which she shares with elder brother Yuzuru means "no sound".
- Hatsune was a contestant of the voting contest Anime Saimoe Tournament 2010 but lost to Natsume in the first round of the main tournament.
- Hatsune has some resemblance to Urumqi Salinas from "The Melancholy of Haruhi Suzumiya."
- Hatsune's first name is also the last name of a top Vocaloid, Miku Hatsune (初音ミク Hatsune Miku)
- In Angel Beats 1st Beat - if the player plays Iwasawa's route, Otonashi never regained his memories and so he never consciously remembers his sister. However, both he and Iwasawa agreed to call their child Hatsune, when he suggested the name for their daughter, indicating that at least subconsciously he does still remember her.
- Her Japanese voice actress, Mai Nakahara, also does the voice of Nagisa Furukawa in Clannad, another work by Key.
- ↑ Angel Beats! Episode 07