Pierrot was the Japanese animation studio that worked with Studio Mir on Book Two of The Legend of Korra.
Book Two: Spirits originally faced a production change when Studio Mir, which had animated the first season, initially declined to continue because of the demanding workload. Pierrot was brought in to handle the animation, but Studio Mir later returned to help complete the season.
The work was divided between the studios rather than being produced entirely by one of them. Pierrot animated episodes 1 through 6 and episode 9, while Studio Mir handled episodes 7, 8, and 10 through 14. This division is why both studios receive credit for Book Two.
The other options are Japanese animation companies, but they were not the studio paired with Mir on this season. Book Two premiered on September 13, 2013, and expanded the show's world through spirits, the origins of the Avatar, and the conflict involving Unalaq and Vaatu.