Kotarou has been working hard to study and find a cure that could heal Yui's illness. He begins to think it is thanks to them that he found a treasure important enough to him to care for. Yui, however, falls in love with Kotarou and she lets him know of her feelings but Kotarou, even though he sees her as a girl most important in his life, knows his love for her is that of a love for a younger sister and thus he decides to let the confession slide to try and not ruin the relation he has with the twins. Due to that he delayed his response and before he knew it, Yui had died. Now in present time, Kotarou still feels as though he has no right to love and that whomever falls in love with him will be sad thus he feels as though that which becomes precious to him will soon disappear. (thetvdb.com)