水銀灯 (Mercury Lamp), first of the Rozen Maiden dolls.
In an episode of the TV series, she makes a rather unusual remark when, while taunting the other dolls, she asks them if they are "taking their lactic acid bacteria" (in the form of probiotic drinks), implying that they are weak and that nutritional deficiencies might be the reason.
Since this joke sounds strange and out of character for her, fan artists have turned it into a meme.