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Yukkuris, named after their catchphrase, are creatures that appeared in Gensokyo one day. Nobody knows why or how. They are grossly misshapen heads with exaggerated facial expressions that bear striking similarities to Touhou characters, constantly telling people to "take it easy". There is a yukkuri for nearly every Touhou character, each with its own quirk and personality. However, all the yukkuris are incredibly stupid, so the quirks and personalities just end up as jokes rather than anything that even vaguely commands respect.

The yukkuri population in Gensokyo has climbed exponentially since their introduction. They're seen as pets, livestock, food, beanbag chairs, pests, and/or targets by the people of Gensokyo. The purpose of the yukkuris remains a mystery and probably will forever, so the best course of action would be to sit back, relax, and take it easy.

They consist of a grossly exaggerated head with hair and head accessories.
Normal ones are about the size and weight of a basketball. However, they can grow to much larger sizes. There are also fat ones which look similar to normal ones, but are slightly bigger and more bloated. If a yukkuri is forced to eat too much food at once, it does not become a fat yukkuri but is instead stretched out and "filled" with so much food that bouncing can force bean paste out of its eyes or mouth. They are filled with bean paste and therefore edible, which makes them a particular target of Yuyuko. Some variants have different fillings. For example, Remilia-types are meat buns.
Their teeth, hair, and accessories are identical to their human counterparts, but somehow turn into bean paste when eaten. It is theorized that they are "grown" with dead cells, much like the dermal layer on human skin. They breathe, eat, see, hear and move (by bouncing off the ground), yet, with the exception of their eyes, they have nothing resembling organs. They are extremely fragile. Severely injuring them is very easy. They seem to make up for it by having lots of stamina. A yukkuri can survive extreme amounts of trauma before dying. Examples include: having a large piece of its body bitten off, having most of its bean paste scooped out, getting all of its skin peeled off, getting impaled on a stick, etc.
They are soluble in water, but only after prolonged exposure. All yukkuris are aware of this and will avoid rain when possible.
Yukkuris are very elastic. However, stretching a yukkuri beyond a limit will render it unable to return to its normal shape, and stretching it even more will break it.
They have a "crying moe", which causes people to abuse and bully them. Even good-natured humans can become sadists. This has a reverse effect on some humans (Sakuya, Eirin and Reimu are often portrayed as having a yukkuri and caring for it), and little or no effect on youkai. In theory, this could be their natural self-defense, a psychic manipulation field that is meant to stop things from wanting to hurt them, since they lack any physical ability to fight or flee from predators. Only, of course, it doesn't work.
Yukkuris are able to regenerate from almost any physical injury as long as they are alive and provided with food (sugar is especially good), but they can still die. Mutant-type yukkuris have an upgraded healing factor. They are somehow able to regenerate without consuming additional mass and do so in a much shorter time frame. Eating a yukkuri whole is one way to counter their restorative abilities, and death is permanent.
A yukkuri’s main goal in life is to “take it easy”. It holds this above all other needs. It is easy to gain a yukkuri’s trust under the pretense of taking it easy. There are variants of “take it easy”; for example, Chen-types “get it”.
They may form "families". These families may be made of one or two species of yukkuris (e.g. Reimu-types and Marisa-types) and the oldest (and therefore biggest) one is the "mother" of the smallest ones (whether they really are its Koyukkuris or not) They are incredibly selfish. They can and will attack and eat each other in dangerous or stressful situations. They will often try to annex human homes. They will claim that the house belongs to them, and that the human should leave. This can be circumvented by asking permission from the yukkuris to take it easy with them. Then, the human can promptly eliminate the would-be invaders without suspicion. They constantly demand food, particularly sweets, from anybody and everybody. Their memory capacities are incredibly small; a yukkuri is incredibly forgetful. If you bully one, there’s a good chance it’ll forget about it within a few hours. There are examples where a single yukkuri eats its family and/or friends and then forgets about it. They retaliate against aggressors through tackling and goring. A common warning sign is a shout of "Stop or I'll get angry." However, due to their extremely low speed, soft bodies, and low mass, the attacks are nothing, but nuisances (however, there are reports of yukkuris injuring humans by biting). The best counter is to simply step on them. Some yukkuris have different defense mechanisms. For example, Iku-types are able to shoot lightning, making them a threat to yukkuri and human alike. Crying, injured, or infant yukkuris tend to slur their speech by adding dakuten (゛) to every hiragana character, e.g. "Yukkuri shiteite ne!!!" becomes "Yugguri jideidde ne!!!" In English translations, it works similarly, like "Take it easy!" becoming "Dage id eajy."
Yukkuris will refer to people they (usually) don't know as "mister" or "lady." This extends to just about everything, leading to phrases such as "mister butterfly" or "mister flower".
Fear and despair makes a yukkuri’s bean paste sweeter. Conversely, emotionally dead yukkuris taste bad. Koyukkuris are slightly sour.
Putting your hand over a yukkuri’s eyes will make it think it’s night.
Even the simplest traps (a box) are too much for a yukkuri to escape.