J. Mitchell was a Starfleet officer in the 23rd century.
In 2285, Mitchell was a commander at Starfleet Academy, where they served as a battalion commander. They were the Director of Educational Services - Starfleet Training Command, with an office located in room #612. Their rank, name, position, and office were listed on a directory placard. (Star Trek II: The Wrath of Khan, set artwork)
This character was only mentioned in writing.
The placard was sold on It's A Wrap! sale and auction for US$31.99. [1]