Monoko (モノ子), meaning "monochromatic child" is a character in the White Desert. She appears to be a young, rather depressed-looking black and white girl.
This unassuming appearance only remains until you use the Stoplight effect on her. She immediately stops moving and changes into what seems to be her true form, which is still recognizable, but horrifically changed: She now has five arms, one of which sprouts out of the top of her head; her stomach grows what seems to be a bloodless wound or vortex; and her left eye and mouth begin to melt or dribble. This is usually interpreted as her being crying and drooling.
If you run into Monoko while she is in this form, the screen changes to black with a detailed picture of Monoko bouncing in random directions while Kalimba music plays.
Monoko can be interpreted as a horribly mangled girl Madotsuki saw in a traffic accident, caused at a stoplight. The head hand could be brains while her arms could have had bones or something to that effect sticking out of them. There are numerous other ways Monoko has been interpreted; one being that Madotsuki sees herself as a horribly ugly person when she wants to be normal.
When she turns into her true form, it is a common meme to exclaim, 「モノ子ッッ可愛いよおかわいいいよォモノ子ォォアアァァア アァッ！！」 or "Monoko kkkawaii yoo kawaiii yoo Monokooaaaaa aa-!!"