Ryu Takeshi (龍武, Takeshi Ryuu) was once a member of Aizen's Arrancar Army, For a very brief period time after defecting from the Soul Society. He eventually left them both because of his unique heritage. If you tought Ichigo was a rare case then you should take a look at Ryu here... The man is a VERY and i mean VERY rare case of a combination between Human Shinigami and Arrancar which gives him a very strange Spiritual Signature and a whole lot of power gained from his unique genetic code.