Promise ( 約束 / やくそく / yakusoku ) is the thirty-third chapter of the Elfen Lied manga series.
This chapter begins slightly before the end of the last one, this time from Kouta's perspective, as he and his father and sister Kanae are staying for part of the summer with Yuka and her mother in Kamakura. Kouta rejects both a tour of the local historical shrines with his father and what he sees as the noisy company of Yuka for climbing the local mountains looking for 'strange animals' to draw.
The familiar story resumes as he once again proclaims how cool Lucy's horns are, thought that she rejects for all the scorn they have brought her. Kouta fails to understand her pain, but empathizes as much as he can, while maintaining how much he likes her horns. Stating that he has to return home to Hokkaido in about a week, he asks Lucy if they can be friends. Stung by memories of the girl who seemingly betrayed her so recently, she rejects this possibility, but Kouta insists he will meet her there tomorrow.
Found by his father, Kouta learns he will be unable to keep his promise for about two days, as the murders of the orphans who tormented Lucy reach the news, with Lucy herself presumed missing and taken by the same murderer. She waits for Kouta and grows despondent, cursing her need to be held and to trust someone again. On the third day, Kouta surprises her and brings a wool cap so she can properly conceal her horns from other people, and holds her hand while they walk and he laughs off injuries he got while sneaking out. Lucy actively fights off her growing feelings for Kouta but promises to play with him the next day.
That night, Lucy's continued detachment from humanity becomes apparent when she showers and raids the refrigerator inside the home of a family she has slaughtered. Rather than be horrified by the corpses she's left strewn about the house, her thoughts center solely on her needing to leave before the police find out the inhabitants are dead, as well as the contents of the fridge expired.
- Given Nyu's lack of maturity at this point in the series, the somber-looking one on the chapter cover seems to be Lucy, perhaps in the present-day, lost amidst the memories seen in this flashback.