The Knight's Letter is a sidequest item given by one of the soldiers stationed at the Great Walls. This sidequest must be unlocked by helping two other soldiers first. These include a soldier standing near the area's Save Shrine who is craving Daea's sandwiches, and a soldier inside the barracks (the area with all the bunk beds) who complains about his comrades' BO and can be given a bottle of Mikan Fragrance.
After completing these two sidequests, one of the soldiers in the first tower (the one who complains about how slow the postal service is) will ask Gale for help and give her the Knight's Letter, instructing her to give it to his mom who lives at Farmer's Block. Although the description claims it's probably filled with sentimental stuff, the opposite is true; upon delivering the Knight's Letter to the soldier's mom, it's revealed that the letter is a message telling the mom not to come to the Great Walls and embarrass her son on his birthday, which is coming up soon. After reading it, the mom gets offended, pays Gale for her troubles, and declares that she will retaliate by not sending her son any more care packages.