To those not knowing who he is, this is the guy:
The Factions Cinematic featuring Shiro can be downloaded here:
http://www.guildwars.com/gallery/video/default.php
First, let's go over what we already know about him:
- Shiro Tagachi was the bodyguard of the 27th Emperor of Cantha. He killed the very Emperor he was sworn to protect.
- 200 years ago, The 27th Emperor of Cantha was attending the Harvest Festival in Kaoya Zun Temple and as he bowed his head down to pray to the gods, Shiro came up behind him and slew him in cold blood in that holy place. The champions of Cantha who were present at the ceremony all rushed to avenge their Emperor, but Shiro was without match in the use of the blade. He slew many of the rampaging soldiers until two champions finally bested him and killed him with the very blades he used to slay the Emperor.
- In his final living moments, overcome with pain, rage and and malice, Shiro let out a terrible cry that caused a Jade Wind to sweep across the lands of Cantha, petrifying the forest and turning the sea into pure jade.
- Prior to his betrayal, Shiro was an honorable man entrusted with the greatest duty a soldier can take on. It is unknown what turned him into a cowardly traitor and murderer
- The tale of Shiro Tagachi in present day Cantha is more of a bogeyman story mixed with myth and a cautionary tale against the dangers of magical power.
We also have hints that he may have returned, but that is unconfirmed.
This is what I have been able to find out about him. Please post here if you have additional information, and I'll update my post.

) on the GW site.