Xuan Wu, 玄武, the Tortoise of the North. Xuan Wu is the fourth Legendary Warrior of Water. Ironically, she came from the Southern Water Tribe yet holds the position of the north according to her Celestial position. She is a healer as well and a warrior. She can waterbend poweful jets that can cut through metal and stone, bend water from the air, clouds, plants, and even bloodbending. She only uses bloodbending as a medical practice not a puppetmaster. As a medical warrior, Xuan Wu can bend ice needles and bend a portion of poison inside them, creating the type of weapon that disappeared within time. These poisoned needles can knock out a giant lion-turtle but it will not kill the victim. They are most deadly when hitting the pressure points. Xuan Wu's spiritual talisman is her golden bow, Genbu, 玄武 (hiragana is げんぶ, forged by the Water Spirit, it can strike down an army from the iced arrows that Xuan Wu waterbended. During inpack, the arrow realeased a type of a small shockwave, knocking out the soldiers within seconds.
Xuan Wu can also bend ice barriars that are so cold that it can't be melt easily by firebenders.
When staying on Avatar Island, Katara is excited to learn advance wterbending from Xuan Wu, although a bit jealous at first, she is easily eagered to learn from her Southern heritage and dedicate herself to teach it to future waterbenders after the war.
Xuan Wu is the Chinese name of the Northern Black Tortoise in the Asian Four Symbols, while Genbu is the Japanese name...it's written in the same way!