A Shady Customer
You will be using your Sheikah Slate to take a photo, so you will have to have completed the "Locked Mementos" main quest. Learn more about it here.
Talk to Hoz at the East Akkala Stable. He is dressed like a soldier. He wants you to go find a suspicious person named Kilton who runs a business called Fang and Bone.
If you have not met Kilton yet to unlock the ability to buy all sorts of neat monster-related apparel and items, go to Skull Lake at night. He's on the eye of Skull Lake that's level with the water. After you're done talking to him, he will teleport away.
From now on, you can find Kilton near any village at night. However, you have to wait for nighttime before teleporting. If you wait for the day to pass into the night at a village, Kilton will not show up. Simply take a picture of Kilton with your Sheikah Slate and show it to Hoz to complete the sidequest.
Note: Kilton's shop is the place where you can get an armor set that turns you into Dark Link! It's expensive but definitely worth saving up for if you want to look a little less heroic.
Hoz will give you a silver Rupee worth 100 Rupees as your reward.