NPCs

In the game, there are currently 4 NPCs Separated into Hostile and Trader type.

Trader NPC
Currently, there is only one Trader NPC, that is called 'Shop'.

You can find the NPC in the Merchant's Village along The "Big" Path.

You can trade 20 Jewels for 1 Apple that heals 20 Health or 100 Jewels for 1 Spin that can be used to spin the Roulette for magic types. Spins are kept even after starting 'new game'!

Hostile NPCs
Currently, there are 3 Hostile NPCs:

Natsu
Natsu can be found in the Nest with a respawning rate of 900 seconds or 15 minutes after being killed. Named after the Main Character of the Manga/Anime 'Fairy Tail'

Upon death, Natsu gives 500 XP and anywhere from 125 to 175 Jewels.

big_thonk
big_thonk can be found in the Waterfall next to AviSompoii. Named after the games' Scripter

It has a respawning rate of 550 seconds or 9 minutes and 10 seconds after being killed.

Upon death, big_thonk gives 300 XP and anywhere from 75 to 105 Jewels.

AviSompoii
AviSompoii can be found in the Waterfall next to big_thonk. Named after the games' Builder

It has a respawning rate of 500 seconds or 8 minutes and 20 seconds after being killed.

Upon death, AviSompoii gives 250 XP and anywhere from 62.5 to 87.5 Jewels.

Source
According to big_thonk the formula for Jewels dropped is XP/4+(0-XP/10[Anywhere between 0 to XP/10])!