Korra

Avatar Korra is Aang's immediate successor as the Avatar. She is a teenage girl native to the Southern Water Tribe and is described as being hotheaded, rebellious and "ready to take on the world". Having already mastered Waterbending, Earthbending and Firebending, Korra only needs to learn and master the art of Airbending and gain full control over the Avatar State in order to become a Fully Realized Avatar. About seventy-five years after the end of The War, Korra journeys to Republic City seeking the tutelage of Tenzin, the son of Aang and Katara, as her Airbending master. During her travels, she discovers anti-bending unrest in the city.

Background
Korra was born in the Southern Water Tribe sometime after Aang's death. Not much is known about her early life, however Korra learned she was the Avatar and began to train in order to become a fully realized Avatar, even mastering Waterbending, Earthbending, and Firebending. Korra still needed to master Airbending, so she journeyed to the crime ridden Republic City so she could learn and master Airbending from Tenzin, Aang and Katara's son. Korra will also need to defeat the Anti-Benders, an organization of people against the art of bending.

Rumors & Speculation

 * Korra copy.png recently leaked unconfirmed line test animation from a Korean studio shows what appears to be Korra battling several opponents. In this, she exhibits mastery over several fighting styles, and performs moves similar to those of previous characters.
 * In this clip, Korra is seen incapacitating an enemy using a technique similar to Chi Blocking.
 * Korra is also seen performing the same move Zuko used to destabilize Azula during their Agni Kai.
 * Aang will mentor her from the Spirit World, just as Roku mentored Aang.
 * This seems highly likely if Aang's own experiences are any guide. Aang immediately felt a strong connection with a statue of Roku, instinctively knowing Roku's name and that Roku was his previous self, and Roku appeared to Aang more regularly then any other of his previous selves.
 * Also, since Aang was the most recent Avatar before Korra, any of his wisdom will be the most relevant to Korra, and the time periods in which they kept balance are very close to each other.
 * It is possible that she was raised in the Southern Water Tribe, but was sent to the Northern Water Tribe to learn Waterbending when she was identified as the Avatar, due to the fact the Fire Nation virtually eradicated Waterbenders in the Southern Water Tribe. However it is also possible that, thanks to Master Pakku's efforts to rebuild the Southern Water Tribe, she was raised and taught in the Southern Water Tribe.
 * It is also possible she was taught Waterbending by Katara.
 * Korra is likely between 16 and 19 years old judging from the picture and the tradition of revealing the Avatar at the age of 16. As the series is set 75 years after The Last Airbender, this would mean Aang died somewhere in between 156-159 ASC and only lived to be biologically 69–72 years (chronologically 169–172 years).
 * The book The Art of the Animated Series depicts several early concepts of a large Polar Bear Dog. The notations describe the creators' love for the idea of the dog, but also states that they were unable to use him in the first series, but then go on to hint that, "We have a feeling he'll turn up somewhere," hinting that the Polar Bear Dog will make an appearance in Avatar: Legend of Korra as Korra's animal guide.

Trivia

 * Korra is a word of Greek and Irish origin meaning, "from the mountain valley." It also means "maiden" in Greek, and "grass" in certain Aboriginal (Native Australian) dialects.
 * Korra is pronounced the same as the Irish word "cora" which translates as "weir."
 * Korra is also the name of a village in the Tibet Autonomous Region of China.
 * Her name follows a long line of tradition in the polar Water Tribes, where often people include one or two "K"s and have an "UH" sound in their name (Katara, Hakoda, Sokka, Pakku, Kuruk, Kanna, Arnook, etc.)