When I Return Home My Girlfriend Left Me A Break-Up Letter

When Kaito returns home from his camping trip he notices something odd, Chizuru is nowhere to be found until he sees the bow with their memories together and a letter, He reads the letter that Chizuru wrote down saying she wanted to break up cause she assumed that Kaito was cheating on her with her colleague Saki. Kaito was confused about what Chizuru wrote about cheating on her, He immediately called Chizuru and explained to her that he didn't cheat on her but Chizuru didn't believe and immediately ignored him and blocked him, Kaito needed to figure out who told her a lie that he was cheating on her