Wryna's Missing Daughter
Immediately after you put the Ruby Tablet in the Chamber of the Goddess, you'll go outside and see Wryna looking for her daughter, Kukiel. She asks you to look for her too and you can speak with various townsfolk about the girl's whereabouts as well.
Orielle's Rumor (Optional)
Over near the houses to the east, you'll see Orielle, the girl with the brown hair dressed in white, orange and maroon. She's got a thought bubble over her head and you can optionally talk with her about a wild rumor she heard. She says there's a dude with a crazy story about some monster kidnapping kids and once she realizes you might think it's true, she tells you where to find the guy and hear the story for yourself.
Rusta's Monster Story (Optional)
Rusta can be found in the Lumpy Pumpkin at all hours, grumbling that no one listens to him or takes him seriously. It doesn't take much to get him to spill the tale about how he saw a monster in the graveyard in Skyloft, as well as explains in detail how the demon entered the storage shed.
Now that we know what to do, sleep in any bed in Skyloft to make it night, then go to the graveyard in the southeast corner. Go hit the tombstone closest to the tree to make it start glowing, then push/pull it. This will make the storage shed door open. Enter, go down the ladder and follow the ramp to the end. Enter the little building found here.
The Misunderstood Demon
Oh my! A big scary demon, who turns and roars at you! You can see the kid nearby, but she doesn't seem frightened. In order to progress, you need to slash the demon with your sword, which causes it to cower in fear and apologize profusely. He introduces himself as Batreaux, a well-mannered, kind-hearted monster who wants nothing more than to live in peace with the citizens of Skyloft. But everyone's afraid of him! This little girl isn't though, so they were playing a game of roaring and screaming. Yeah, cause that'll help things, now won't it? Anyway, he didn't mean to worry her parents, so she'll head home now and be safe and sound in the morning.
He goes on to explain that when humans help other humans, they can "form a substance known as Gratitude Crystals," which have the power to do amazing things! Batreaux is looking to collect these Crystals and hopefully, one day be able to use them to transform into a human himself. You wouldn't be willing to hunt them down for him, would you? Oh, that would be so splendid! Your efforts are sure to be rewarded!
This quest unlocks the remaining Gratitude Crystal side-quests, some of which have other requirements before they become available. You can now find bunches of five Gratitude Crystals by helping people, but also 15 individual Crystals can be found laying about the world above the clouds. See the full guide for locations of all the Gratitude Crystals.
The Happy Mother
Return to the surface and head to Kukiel's house. Sleep in a bed until morning, then speak with her mother, Wyrna. She's so relieved that her daughter is alright! Though my, she has some strange stories to tell... This will grant you your first five Gratitude Crystals! You can head back to Batreaux to get your first reward from him now, if you wish.