An accessory for the VTuber Tsukumo Sana, who almost always wears it on her head like an ahoge. It's a thin, light-purple piece of material that sticks out from the top of the head and then spirals outward, similar to a mosquito coil. If a post clearly shows Sana not wearing her limiter, it's probably a case of the no headgear tag.
Lore-wise, it's meant to shrink her down to a size where she can safely interact with humans, since her true form is almost 1.7 billion metres tall and would make it impossible for her to be anywhere near Earth (which is only about 12.7 million metres wide) without immediately destroying it through sheer gravitational force.
