To start the mission, I'd need a tactical master. For this role I nominate Shikamaru Nara. Though he's no L Lawliet or Light Yagami, Shikamaru is able to plan out the enemy's next three dozen moves. He's a quick thinker in tight situations and has his own Shadow Possession Jutsu if need be.
Who other to sneak into high security and steal something than Garett, the main character in the Theif games. He, too, can worm his way out of trouble. Armed with an array of arrows, he can keep himself in the darkness, take out who he needs to, and escape as if he were never there. His claw can get him up the walls of Fort Knox and into the building. Then with his mastery in lock-picking, safe-cracking and switch-finding, he can easily get through doors and other obstacles. Finally, escaping the fort, his trusty rope arrows will help him swing to safety.
Burter (Dragonball Z)
In the unlikely event that the mission is compromised and Garett is down, Burter is here to come to the rescue. Though a big, hardly stealthy brute, his speed is far too impressive to be noticed by humans. The big blue Burter would speed through the doors Garett left open, with brute strength could take out who he needs to get Garett and the secret documents hidden in the fort, and leave faster than he got there.
If everything goes as planned, Garett is out of the fort with Burter waiting for him. With Garett on his back and the documents pocketed away safely, Burter rises up and flies away quickly.