Love Sign

During her summer break before entering high school, Mimori Ichigo goes to her grandmother's house near the ocean. There, she helps out at her grandmother's shop. One day while walking her grandmother's dog in the evening, she meets a boy named Kouki. They meet again, but this time she also meets Kouki's cousin named Taiga. For some odd reason Kouki has no memory of when they met and Taiga begs her to not tell him either. Later, she and Kouki sneak out to watch the fireworks, but they separate dramatically. When Ichigo goes to high school, coincidentally, Kouki and Taiga are also there, but they have no memory of her. Are they faking it or are they just lying? If so, why lie?