Final answer:
Aang, the Avatar, is the last known keeper of energybending in the Avatar: The Last Airbender series.
Step-by-step explanation:
According to the Avatar: The Last Airbender series, the last known keeper of energybending is Aang, the Avatar and protagonist of the series. In the third season finale, Aang successfully learns the ancient art of energybending from the Lion Turtle, a massive spirit creature. He then uses energybending to strip the villain, Fire Lord Ozai, of his firebending abilities instead of killing him.