That is an excellent question for anyone who does not know anything about touhou. I'll keep this simple, though it shouldn't matter to any yousei reading this. .... assuming any of them could.
Yousei, simply put, are the fairies of Gensokyo. They are embodiments of things that happen in nature. Perhaps the best known of them is Cirno, who is an ice fairy, though there are thousands upon thousands of the little bugs flying around all over the place. They're bloody everywhere, and doing all sorts of things, all the time. The thing is, though, that they have a problem.
Most yousei, you see, are stupid as hell. Thoughtless. Witless. Careless. The gamut of 'not very bright at all, miracle they're alive'. And, well, it is, after a fashion. Yousei tend to get smacked around a fair bit, and killed. However, as they're the embodiment of natural phenomena, they cannot die for good. They simply reappear a few seconds later, no worse for the wear, though they do remember what happened.
In Lainy's case, though, she's the embodiment of the sound of raindrops falling. Her name could be taken to mean Rainy Cloud... which is perhaps a bit imaginative for a yousei. Then again, Cirno, if you noodle it around in an Engrish translator for a bit, could be taken to mean Snow. Think about it, eh?
Lainy, though, is a simple sort, much as all other yousei are. She enjoys the simple things in life... good food, good fun(not necessarily clean, healthy fun), and warm, soft places to sleep. ... this has gotten her in trouble more than once, because one of the softest places she likes to sleep is a Human or Youkai's underwear drawer. Don't ask why, she may not even know herself. Some people suspect it's because of that one time she fell asleep after getting locked in one by accident, and woke up when the woman put on the bra containing her. Sanae was a little miffed at her after that one, though.(this was when Lainy was about a quarter of her current size, or less, though).
Still, you get the idea. She's not a very bright girl, Lainy, but she's got some sense to her. Some. A very little. She knows to listen to some people because they're smarter than her, so if they tell her that something's a bad idea, she'll probably not do it.
Still, that's all details. AS far as her abilities go, she's actually capable of causing rain, varying from a light drizzle to a torrential downpour. She tends to look for places that she thinks needs rain, though, before doing this. However, after watching some Kappa one day, she may have gotten the idea that gloomy people need a small raincloud over their head... or that two people who are soaking wet and of the opposite gender might have the hots for one another afterward. Cartoons were a terrible thing for her.
Of course, it probably doesn't help that Sanae told her that the best way to get someone's attention is to yell 'Hey! Listen!' at them as loud as she can.
Physically, though, Lainy stands out a little bit. Her hair is a blue-green color, coming down to her butt, worn in a french braid style, leaving it right in the middle of her six wings, while her dress is a simple, blue-gray one, with stockings and little shoes. It's best described as a sundress, though the irony is lost on her. Around her neck she wears a tiny locket, and she tends to have a snack somewhere on her person at all times. After all, it's best to always be prepared. She can't remember who told her that, but it seems to have stuck. And, well, she's actually fairly tall for a yousei, standing at two feet tall.
Secret Achievement Unlocked! Gensokyo's First Ballistic Fairy, 5000 points.