Orange: The Complete Collection 1 by Ichigo Takano
"Everyone has regrets in life. So who wouldn't take the chance to change the past if given the opportunity? When sixteen-year-old Takamiya Naho receives a mysterious letter, claiming to be from her twenty-seven-year-old self, her life is suddenly thrown into flux. The letter informs her that a new transfer student by the name of Naruse Kakeru will be joining her class, and to keep her eye on him. What is Naho to make of the letter's contents and its cryptic warning?" -WorldCat
Orange: The Complete Collection 2 by Ichigo Takano
"Naho immediately feels a connection to the new boy who has transferred to her class. Kakeru is calm, kind, and seems to like Naho as well. But their relationship gets thrown for a loop when Naho receives a letter from the future. The letter explains that Kakeru will die unless Naho does exactly as the letter says. But changing fate is no easy task. When the letter starts to get things wrong, Naho worries she will lose Kakeru forever. Luckily, Naho has her friends to back her up. Not only do they want to see Naho and Kakeru get together, they also have time-traveling letters of their own." -WorldCat