Secret pandaren

One pandaren brewmaster shared a separate view from most of the pandaren who had been hired by the Alliance in order to fight the blood elves. These pandaren mercenaries tried to keep blood elves from getting to Outland. However one lone pandaren did not share the opinion of his brethren and chose to join Prince Kael'thas and Lady Vashj when they broke out of the dungeons and went through a portal to Outland to escape the Alliance. This pandaren assisted the heroes in rescuing Illidan from enemy forces.

Information
If you find the secret sheep area in the Blood Elf Campaign during the mission where you have to help the Blood Elves escape Dalaran Prison, you will unlock a secret Tower Defence stage where you have to build unique tower to prevent Garithos' men from destroying the portal to Outland.

If you win that stage, you will be given a random Pandaren Brewmaster for last few levels of the blood elf campaign. In the first level he appears in, you can only control your heroes and are suppose to escort Illidan (trapped in Maiev's wagon) back to your base.

Having the pandaren brewmaster in this stage makes things a lot easier.

Speculation
It is possible that the pandaren isn't part of the story as he hasn't been mentioned since.