Heart no Okurimono

Shiro had a weak heart, so she had been bedridden since she was little. A girl from her neighborhood named Kuro would always visit her and keep her company throughout the years. Eventually, Shiro's condition got to the point where she needed a transplant, or else she would die. Kuro then offers her heart to Shiro, and has hers replaced by a mechanical one. This story is about them adjusting to their new hearts and to their feelings for each other.