After beating the Skyview Temple and returning to Skyloft, a lady approaches Link, asking if he can find Kukiel, her missing daughter. Some citizens of Skyloft say that she was last seen around the graveyard, but I haven't figured out what to do there. How do I find her?
From this walkthrough:
If you ask the people in Skyloft, you’ll learn that Kukiel was seen playing near the graveyard. If you head towards the graveyard you’ll meet a woman who will tell you that one of the villagers, who is considered crazy, is always talking about a demon that lives in Skyloft. She’ll also tell you where to find him: Pumpkin Landing.
Pumpkin Landing is another island that is southeast of Skyloft. Dash off Skyloft, call your Loftwing, and head that way!
Once you’re there, enter the only building on the island, the Lumpy Pumpkin. You’ll find the old man inside having a drink. Speak with him and you’ll learn that he saw the demon in the graveyard at night. He’ll also tell you how to find the demon!
Head back to Skyloft and sleep in a bed until night. Then, head to the graveyard. Once you’re there, follow the instructions the old man gave you:
Find the gravestone closest to the large tree. This will be the gravestone in the top-left corner of the graveyard. Roll into the gravestone. You can roll by shaking the nunchuk while sprinting. If you do this correctly, a glowing symbol will appear on the gravestone. Push the gravestone. When you do, the door of a nearby storage shed will open and reveal a ladder. Go down the ladder and follow the path. At the end, you’ll find a house and hear a scream. When you enter the house, you’ll find Kukiel and a bat-like monster who will roar loudly when you enter. However, when you try to attack the monster, he will cower in fear and beg you not to hurt him.
The monster’s will introduce himself at Batreaux and convince you that he only wishes to be friends with the people of Skyloft, but due to his terrifying appearance no one – except the child Kukiel – will spend time with him.
Batreaux will then ask you to find Gratitude Crystals for him so that he can become a human. This begins an extensive side quests that is tied to all of the others.
Batreaux reminds you that it is dangerous at night, so Kukiel is staying at his home until morning. He promises that he will then send the child home.
Return to a bed and sleep until morning. Then, head to Kukiel’s home. She lives in the house next to the bridge in Skyloft. Inside, you’ll find Kukiel safely at home. Speak with her mother to receive five Gratitude Crystals.