By Firefly's Light
Lasli is available inside her house (the closest to the entrance to town) at night, after approximately 10 p.m..
The goal here is to let 5 Sunset Fireflies loose inside her house, so first you will have to catch some if you do not already have them. They will fly around Kakariko at night, although typically closer to the midnight through 3 a.m. timeframe unless it is raining. Teleporting away and back can respawn them. They will disappear if they detect you, so you have to sneak up on them. They are also available elsewhere in Hyrule.
Once you have five, release them in Lasli's house while she is there to get your reward, a purple rupee.
Tip: If you have done the Slated for Upgrades Side Quest and have the Sheikah Sensor +, take a picture of a Sunset Firefly. You can then set it as your Sensor+ target for help in finding more of them.