Among the many fairies of Gensokyo, there are a small handful throughout the years who have been able to develop a higher level of power and clarity over other fairies. This heightened level was duly noted by the Hieda family, Gensokyo's eternal chroniclers, and termed "greater fairies". However, just as with any other magical being, power unused wanes, and many of the greater fairies began to "relapse". While there are a number of fairies displaying unusual power in Gensokyo nowadays, the ice fairy Cirno being the prime example of this, only one actual "greater fairy" is generally known, though others might exist.
This greater fairy goes by the nickname Daiyousei, though whether she has forgotten her original name or refuses to tell it is unknown. She inhabits the Misty Lake, near the Scarlet Devil's mansion. A generally friendly presence, she is one of the few really approachable yousei.
Well, what's the point in making everyone hate you, after all?
As one might be able to tell from that statement, Daiyousei is a friendly and generally wise being. Her age predates most if not all well-known fairies in modern Gensokyo, easily several centuries in age. She has also devoted the proper focus to maintain herself at the greater fairy "level", keeping her magic strong and her mind sharp, which is why she has been able to remain a greater fairy for so long. This has also put her leagues ahead of the other fairies in terms of intellect, possibly in part because she's the only one that actually listens when Keine says something.
You shouldn't assume one is stupid just because they're a fairy... Okay, ninety-five times out of a hundred that assumption holds true, but...
As has been said, Daiyousei in fact has a fairly strong and rational intellect, unlike other fairies who blunder blithely into death on an hourly basis.
Cirno, wait a second... if you do that, the shrine maiden will get really upset and come after you...
While she doesn't command quite the respect greater fairies once had, Daiyousei's wisdom and strength do earn her some influence over other fairies. She generally exercises this in trying to keep the others from getting themselves too badly hurt, or from hurting humans with their pranks. Unfortunately, the biggest four offenders generally tend to never listen to her at all...
You three -do- realize this is going to get you three killed... Probably me too, unless Yuuka's in a very uncharacteristic mood...
Of course, like all yousei, Daiyousei is an embodiment of a natural phenomenon (in her case the ripples and currents of the lake's water) and thus cannot properly be killed. Dealing her damage that would be fatal to a normal youkai or human simply causes her to dissipate and flow back into nature, to return once she has recovered. This allows her to be rather nonchalant about being killed, even by something as powerful as a Master Spark.
This doesn't mean she's flippant about getting killed, or that she's completely all right with dashing into a situation where she'll be killed. Fairies still feel the pain when they are "killed", and Daiyousei is no exception; Hieda no Ago put forth the theory, in fact, that greater fairies may be even more sensitive to that pain, being on a higher intellectual level. While being killed doesn't particularly bother her, she won't blithely go into a certain-death situation...
We can't really die, and the more foolish among us have on occasion perpetrated quite lethal pranks on the humans... I don't really expect to have a lot of fans in the village.
Daiyousei, while friendly, has a rather small circle of friends. Most of these consist of fairies, particularly her closest friend, Cirno, and younger, weaker youkai like Wriggle. More powerful youkai tend to see Daiyousei and fairies in general as a nuisance, particularly around incident-time, and humans are rarely fond of yousei. Daiyousei does have one human friend in the form of
Chitose Izumo, and is very fond of her, as well as having at least cordial relationships with the teacher Keine and the shop-owner Rinnosuke. However, such friendships are rare, and Daiyousei doesn't expect most humans to react kindly to her presence. She contents herself with what little she has, though, and is generally quite happy playing with the other fairies and younger youkai.
Hmm... Don't these panties look familiar, Chitose?
That said... for all her intellect and reason, Daiyousei is still very much a fairy. Her pranks are a bit more thought-out and much less likely to put the human targeted in any form of danger, but she's just as prone to pranks as any other fairy... provided neither Mokou nor Yuuka is involved. So don't be surprised if you wake to find your shoes tied together or your chair coated in honey...
Oh...! A visitor? Let me go get some tea... Keep your wits about you; Sunny, Luna and Star are in a playful mood today...
Overall, Daiyousei is a kind soul fond of company, though if she thinks someone might be in danger near the lake she's quite capable of warding them off with a display of danmaku... provided they're not an overly-powerful shrine maiden. Daiyousei is sweet, though, and helpful, always seeking both to keep her "cousins" from causing too much trouble and keeping them from getting blown up. She enjoys meeting new people and hearing stories... and generally always has a cup or two of tea at the ready~
Alternate form...
There are old legends told among the yousei about fairies that were able to attain a truly ascended state of being... ones that gained an existence separate from simply being a "face of nature"... Apparently Hieda no Aichi knew of one, and termed them "shinyousei"- true fairies. But I've only ever known of one fairy who gained such a level of being...