Doubter
Heh. That guy's never going to develop.
The Doubter, referring to Kooby, Chapter 5
The Doubter[1] is a Darkner NPC in Chapter 5 of Deltarune who first appears in the Garden alongside Kooby.
The Doubter is a parallel to the Puzzle Masters. Its appearance heavily resembles a puzzle piece like Jigsawrys. However, the most glaring difference is the lighter baby blue color, and the flower growing on top. It also sports a more calm expression in contrast to the Puzzle Masters.
The Doubter appears to have a condescending attitude, believing that Kooby will never develop and expressing shock when he actually does.
The Doubter is found in the Garden with Kooby, near a tree to the left of the lake where Asgore and Flowery went fishing. Asserting that Kooby will never develop, it expresses shock once Kris "develops" Kooby by watering him using the watering can.[2]