Angst? What Angst?: Setsuna's life was so awful at the start of the series that he doesn't mind the prospect of having his heart eaten in one year by an Oni. In fact he is grateful that he is given the opportunity to at least not be a burden to his sister and give her the money she could use to start a new life that he believes he messed up due to her having to support him.
Epileptic Tree: Many fans assume that before Setsuna's heart is eaten he will get a transplant and be romantically involved with one of the Oni Princesses. Last chapter shows that the Zombie Clan uses a technique to allow him to live and keep the healing qualities of the heart, and the second three of the Oni Princesses and Armia are after him.
One-Scene Wonder: Middleman, who only appears to give Setsuna the contract and is never seen or heard from again.
Setsuna's life is shit. He had a heart so weak that he was incapable of any real physical activity, his parents are dead, his sister couldn't afford his medical treatments so she had to take out loans from evil debt collectors, he was horribly bullied, and so to help his sister he decided he would be better off dead. He then sells his heart to a guy for 10 million yen, about $100,000, in a years time. So he gets one year with a good heart and a happy family before he dies. He is happy with this.
His sister, though she doesn't get as much focus, has it pretty rough too. Especially considering that her brother will be dead in about a year, and no one has told her yet.
