Yu'lon is the spirit of wisdom. She watches over The Temple of the Jade Serpent in the form of a pandaren, observing her visitors and disciples closely. She is very kind and compassionate, as well as being very patient and slow to anger.
As the Alliance and Horde (with the Jinyu and Hozen respectfully) bring war in front of the temple they rouse the Sha of Doubt. In the process, the statue of jade that Yu'lon needed to start a new life cycle is destroyed, killing many.
Even watching all this devastation, Yu'lon assigns neither blame nor vengeance to either side, albeit she is noticeably sad and disappointed. At the end of the Jade Forest questline, she personally escorts the player out into the Valley of the Four Winds, giving advice and encouragement along the way.
Mists of PandariaThis section concerns content exclusive to Mists of Pandaria.
Every hundred years a enormous jade state is built in the temple grounds, so Yu'lon can transfer her life essence into the sculpture, which would then turn into a new being. This cycle has been going for generations.
Yu'lon and the other three Celestials opened the gate to the Vale of Eternal Blossoms.
War CrimesThis section concerns content exclusive to the Warcraft novels or short stories.
Yu'lon and other Celestials chose to hold a trial involving the members of the Alliance and Horde rather than sentence Garrosh Hellscream among themselves. During the trial, Yu'lon took the form of the Panda cub from the Jade Forest as a show of respect to the gathered races.