Tenzin

Tenzin is an Airbender and son of the late Avatar Aang and Katara. Very little information has been gathered about his character, though it is known that the current Avatar, Korra, is seeking his tutelage in Republic City. He will be featured in the upcoming spin-off series, Avatar: Legend of Korra.

Background
Tenzin was born sometime after The War to the Air Nomad Avatar, Aang, and the Waterbender, Katara. At some point in his life, Tenzin was taught the traditions and customs of the Air Nomads by his father and is presumably an Airbender, because the new Avatar, Korra is searching for his help in the crime-ridden Republic City, so she may learn Airbending. And he lives in Repulic City.

Rumors & Speculation
These storyline elements remain unconfirmed.
 * In theory, Korra is the spiritual parent of Tenzin, seeing as how she is the new Avatar, thus being Aang in a previous life. This kind of relationship would be similar to the relationship between Zuko, Azula, Ursa and Avatar Aang.
 * It can be presumed that following Tenzin's birth, Aang was no longer the last Airbender.
 * It is currently unclear if Tenzin will be the only living Airbender in Avatar: Legend of Korra. He is so far the only one who has been confirmed. However, the possibility of Aang and Katara having other descendants, also potential Airbenders, has not been ruled out.
 * The marrying age in the Avatar World, at least within the Water Tribe, is 16 (presumably for both parties). Thus, as the series is set 75 years after Avatar: The Last Airbender, with Aang having died sometime between 156 and 159 ASC, Tenzin's maximum possible age is either 71 or 72.
 * Tenzin being 71 at the oldest seems more likely as Aang was only 12 years old (biologically) at the start of the first series and only 9 months worth of time is clearly shown to have passed during Aang's journey, as both solstices occur within the show.
 * Tenzin's minimum age is much harder to estimate, though the lowest that seems plausible is 41.
 * However, this would mean that Katara would have given birth to Tenzin at the age of 51, which is the average age most women start menopause. Thus, it is likely that he is older.
 * He may know relatives or descendants of Aang's friends.

Trivia

 * Tenzin is the first canonical interracial child ever mentioned in the franchise.
 * Tenzin's name is possibly derived from the 14th Dalai Lama, Tenzin Gyatso, also the inspiration for Monk Gyatso.

Relatives

 * Aang (Father)
 * Katara (Mother)
 * Sokka (Uncle)
 * Hakoda (Grandfather)
 * Kya (Grandmother)
 * Kanna (Great-Grandmother)
 * Pakku (Stepgreat-Grandfather)
 * Korra (Spiritually)