Mujin

is a shinobi from Sunagakure and a teammate of Sajin and Mijin.

Naruto's Back: Friends' Tracks
Shortly after Gaara was made Fifth Kazekage, Mujin's team went to join in the early Chūnin Exams co-sponsored by Sunagakure and Konohagakure. For the first exam, the teams were all split up to take the written test in separate rooms. Shikamaru Nara as proctor explained that everyone only needs to answer one question on the test but their combined score with their team-mates will determine if they pass or fail the exam as they need exactly 100 points to pass. While everyone began to panic on how to pass the test, Sajin contacted Mujin and Mijin with his Sand Characters, telling them which question each should handle.

After everyone answered and time was up, Shikamaru revealed a bonus question. The question asked who the team would sacrifice in a situation that requires a decoy, noting that while the answer had to be unanimous to pass, that the person picked will be disqualified. He and Sajin decided to pick Mijin and sent the message to him. With the end of the first exams, he and Sajin moved outside where those who passed were gathered. Temari, the proctor of the second round of the Chūnin Exams, exclaimed that those who don't have all members of their team are disqualified since you weren't suppose to answer, leaving the bonus question blank. With that he and Sajin aren't allowed to go forward with the others.