Monoko (モノ子, "monochromatic girl") is a character in the White Desert. She appears to be a young, rather depressed-looking black and white girl. Her name comes from the Picture/モノ子.xyz file from the game, and is thus one of the three characters with an official name other than Madotsuki and Monoe.
This unassuming appearance only remains until you use the Stoplight effect on her. She immediately stops moving and changes into a form which, though still recognizable, is 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 crying and drooling.
If you run into Monoko while she is in this form, a Full-screen Event activates to black with a detailed picture of Monoko bouncing in random directions while Kalimba music plays, surprising the player.
A famous meme in the Japanese fanbase is the exclamation 「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-!!" (see Nicovideo page for Monko, and example of usage in comments on a video). This meme originated in comments on "let's play" videos on Nicovideo.
Interpretations
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.