[ Warning: Some links in this entry are slightly NSFW ]

About Sakuya izayoi

Sakuya izayoi is one of many characters from the Touhou series, always wearing some kind of a maid outfit.
Here is an extract from that character's TouhouWiki description :

Easily one of the most enigmatic characters, despite appearing in so many games, Sakuya is the Chief Maid at the Scarlet Devil Mansion. She works for her mistress, Remilia Scarlet, and is apparently the only human working at or living within the mansion. She has the power to stop time. She is known to then place knives in midair and resume time to allow these knives to fly towards her targets. Stopping time is also a handy way of doing large amounts of maid work in a short time. Because nearly everyone living or working at the mansion are maids, being the chief of them all means there are few people there with more authority within the mansion than Sakuya.

Sakuya first appeared in the 2002 the Embodiment of Scarlet Devil, 6th game in the Touhou series, as a boss who has tendencies in stopping time and throwing knifes at the player, two main features that will become characteristics of her persona and of her further depictions. She has then been a recurring character in the following games.

Meme's context : Immaterial and Missing Power

Immaterial and Missing Power is the 7.5th games in the Touhou series, released in December 2004. It was called "7.5th" mainly because, instead of being the sole work of ZUN and Team Shanghai Alice, it was also a collaboration with Twilight Frontier which did most of the work in that peculiar game.
This opus was very unique in the way that, contrary to the previous and following games on the same theme which were called danmaku or bullet hell, a peculiar type of shoot'em up game, Immaterial and Missing Power was a 2d fighting game.
In it, what will strike fans and give birth to a specific meme isn't the game itself but this particular portrait available of Sakuya Izayoi, done by Alphes and not ZUN as he usually would :

Breast padding: Birth of a meme

In all the other Touhou games (with the possible exception of Embodiment of the Scarlet Devil says TouhouWiki about Sakuya), ZUN drew the Sakuya's character with relatively small breasts, as the following picture shows:

This sudden change in breast size, that only remained for one Touhou game, let the fans with a rather startling hypothesis: Sakuya must have used breast pads to pretend having a larger chest.
From that point, many fanarts started to be made on that sole theme, replacing her knives with pads or drawing her caught in the act by other Touhou characters and defending herself by stating that her breasts are natural:

MADs = PADs

Due to the huge communities behind the Touhou fanbase as well as the constant reuse of Touhou characters and musics in MADs, as proven by the U.N Owen was Her? phenomenon (and many more), MADs made on that Touhou's inside joke were practically inevitable.
Starting the 12th of November 2007, the first MAD dealing with Sakuya and pads was made on Nico Nico Douga and was called 咲夜は大変なPADを盗んでいきました (Sakuya stole a precious PAD).
Further Mads will then be posted to Youtube, as well as original video creations dealing with the meme:

The PAD's Sand Canyon, derived from the Sand Canyon Mads, would be reused in some other MADs as well:

There were so much MADs made on Nico Nico Douga that the meme got its own article there roughly named PAD (translated by Google translate)

Maid! Knee socks! Pad-Chief!

In 2008, a doujin circle named Innocent Key, well-known for remixing Touhou music, did their draw based on the Sakuya's breast pad legend. The result:

The recurring lyrics are saying メイド！　ニーソ！　パッド長！ ( Meido! Niiso! Paddo-chou! => Maid! Knee Socks! Pad-Chief!) and have become the Sakuya character's nicknames widely used by fans.

The following MAD made on it has been named "PAD" as a pun:

Knifed

On a related note, a small westerner trend on the meme appeared rather recently (at least 2009) and consists in a Sakuya's version of Candlejack. Because the character is well-known for throwing knives at people, some fans usually write themselves as being cut mid-sentences while talking about Sakuya and pads or when simply saying something mean about her. They also add "knifed" next to their message:

Youtube video based on that trend:

Related link

• Sakuya's lecture on PADS