Painter-kun (Also referred to as Kamen) is a character found through a painting in the Art Gallery, or through an area of the Red Brick Maze that you are sent to by interacting with one of the Witches in the Day and Night Towers.
Interaction
If Urotsuki interacts with him while he isn't doing anything, he will simply look at Urotsuki, silently.
If you kill him, him his blood will be splattered over his art, his mask will fall to the ground, and he will come back after a week (in-game week). This unlocks Kura Puzzle #28 .
Appearance
He has blonde hair and wears a multi-colored shirt/smock, a black, dog-like mask and jeans. The only paint color that is seen on his brushes is red.
Others
Painter-kun is found painting a large mural of something that looks like a huge dinosaur (see next paragraph). He is busily painting and never seems to have enough time to talk to you.
The dinosaur in his painting highly resembles the one you can see in The Zoo in Rainbow Silhouette World. Both of these areas were conceptualized by the same person - 909 of the original production thread - and as such, the resemblance is intentional. (However, Rainbow Silhouette World was not added by the time 909 posted the art for either of these areas, and as such they purely conceptualized The Zoo itself without ever knowing that it would be added to Rainbow Silhouette World.)
Directions
The Nexus → Garden World → Blue Forest → Art Gallery