Haru-oboro (a Touhou LW original) refers to thick fog in spring. This is an apt temperament for Koakuma, a creature summoned from Makai with too much power. Koakuma's temperament has been altered with magic, leaving only her strengths and weaknesses to the Three Essences due to their influence over the astral realm.

The ability of Koakuma from the world shrouded in overly scarlet mist (a Touhou LW original). Like our world's Koakuma, she has kept her ability hidden. Despite the Vampire Incident being the first-ever incident in Gensokyo, there are no more than passing mentions of it in any of the trustworthy historical records. All that is known for sure is that the incident was caused by a vampire (or vampires) who came from outside Gensokyo. Half of Gensokyo's youkai sided with the vampire, and the other half opposed her, with the opposing side eventually winning out. The events of this incident directly led to the creation and implementation of the Spell Card Rules. All the reports on the prolonged Scarlet Mist Incident (the 6th prolonged incident) are of dubious quality. And despite the connection between incidents, they do not shed much light on the Vampire Incident. However, they do hint that the being known as Koakuma might have played a part in both. Note that Koakuma means little devil. It is not her true name but merely the name of her species. This makes it hard to ascertain whether the Koakuma referenced is the same person, but it seems likely since they are both said to be a familiar presence within the Scarlet Devil Mansion's library. Also, as her name implies, she isn't very strong, meaning she wouldn't normally be suspected of causing or prolonging an incident. She is still a devil, though. Devil is a fairly broad term that appears to include vampires, youkai magicians, and beings from Makai. However, it seems that not all beings from Makai are devils. They can be categorized as follows: - Devils: Beings of pure magic, majin that have lost all semblance of their former humanity, and magical beings that more closely resemble beasts than humans. - Majin: Humans that have become beings of magic as well as magical creatures that share many traits inherent to humans. - Demons: Another name for both devils and majin. - Makaijin: Beings that look like humans who either live in or come from Makai. Since the realm of Makai distorts all who live there, they will eventually become either majin or devils. All of these beings are inherently tied to magic. What makes devils particularly interesting (and dangerous) is how their existence is sustained by magic. Instead of the cells that make up human organs, nerves, and muscles, devil bodies are composed of magical nanomachines arranged in circuits. You can think of these nanomachines as magical cells capable of casting their own spells. It isn't exactly clear what role this Koakuma played during the Vampire Incident. However, it is known that one of the vampire's subordinates summoned all kinds of magical creatures to serve as guards and soldiers. The Scarlet Devil's honor guard was composed of youkai possessed by devils, and they were all bestowed the title of "knight." But Koakuma is an exception. She did not possess any youkai. Instead, it's possible that she assisted with the possessions because she was summoned from Makai with more power than any of the other devils. Either way, this additional power let her manifest with a form closer to that of the true form she left behind in Makai. Apparently, the form she has now closely resembles the form she had back then.