Donboy is the elven/pixie boy from episode 17 "A Small Problem".
The mysterious Donboy who wrote everyone a message in the final episodes telling them to go rescue Hinano and Co. was none other than the elven/pixie boy that once befriended Truffle as a child, and again saved them when the gang got miniaturized. In "A Small Problem," it is revealed that this boy has been following Truffle and keeping an eye on him in secret after befriending him (however having to wipe his memory after each time they meet). With Truffle being tortured and Hinano's group stuck in a deep dungeon, he'd certainly want to do all he could to save them.
