Cawlin's Letter

This is a side quest in Skyward Sword that is available after you receive the Harp. After completing this side quest, you will obtain 5 Gratitude Crystals, no matter your decisions.

Cawlin's Love Letter
Speak to Cawlin near the Knight Academy's lower entrance during the day.He will give you a love letter intended for Karane.However, Link will consider giving it to the ghost haunting the restroom, too.It doesn't really matter if you give it to Karane or the ghost, because you will receive 5 Gratitude Crystals either way.

Giving the Letter to the Ghost
At night, give the love letter to the Ghost in the restroom.Skip to morning by sleeping again and explain to Cawlin what you used his heartfelt letter for.Afterward,sleep until night again and visit Groose's room.The ghost will be haunting Cawlin but it is willing to give you 5 Gratitude Crystals for your help.

Giving the Letter to Karane
Karane is in the classroom on the first floor of the academy.Deliver the letter to her and she'll read it out loud to you, along with her personal opinions.Next, find Pipit which is in the hallway of the second floor of the academy, and tell him about the love letter. Run on down to where Karane is and watch the cutscene.Now that Pipit got what he wanted, he'll unknowingly give you 5 Gratitude Crystals for unknowingly hooking him up with Karane.