Yung was the Captain of King Bumi's army. He was with King Bumi when he decided to surrender Omashu to the Fire Nation. After the enemy took control, he became leader of the resistance trying to free the city. He later retreats from the city with his soldiers and the citizens.
Yung is a gung-ho Earthbender who is willing to die rather than turn Omashu over to the Fire Nation. Yung led the resistance, planning attacks and trying to remove the city's rulers from power, in hopes of driving the Fire Nation out and reclaiming Omashu for his people.
When Aang and his friends get chased by Mai, Yung and the others help him escape. Once down in the tunnels, Yung proceeds to explain what is happening to the Avatar. Aang tells Yung that is is best to retreat to fight another day. They agree and follow Sokka's plan. They get pentapi and use their prints to act like they have pentapox. After they get out, he notices they have an extra person after he did a head count, Tom Tom. He hands him over to Aang who takes him back.
It can be assumed that Yung and the other citizens moved back into Omashu after Bumi liberated the city.