Grand Marauder Sai is a human located at the Silver Enclave in Dalaran. [34.6, 69.3] Sai wears the Tier 2 Set Bloodfang and wields Ancient Qiraji Ripper and Maladath, Runed Blade of the Black Flight. (Not Maladath, looks same but its a quest reward from opening of ahn'qiraj quest line. http://wowwiki.wikia.com/Ravencrest's_Legacy )
Sai was a Blizzard employee who played a rogue on the Tichondrius realm, she was quite famous for her high PVP skills. She stopped playing her Rogue and it was assumed Blizzard added her as an NPC in WotLK because she did not expect to play her rogue again.