Cadet Karl-Willem Brandhorst was a male Human in the 24th century and was Gary Mitchell's roommate at Starfleet Academy between 2251 and 2255.

Brandhorst took his place at the Academy very seriously, and would often advice Mitchell to take his place seriously as well, in an attempt to control some of his actions. One notable example was Mitchell's first meeting with James T. Kirk, in which Mitchell referred to Kirk as a walking stack of books, to which Kirk overheard.

When Mitchell was invited to participate in a training mission aboard the USS Republic by Admiral Pearson Everett, Brandhorst couldn't believe that he, someone who never stopped studying, was left out over Mitchell, who used the workstations to check the schedule for the women's hypergravitational aerobics classes. (TOS - My Brother's Keeper novel: Republic)

By 2265, Brandhorst was the chief science officer on the USS Potemkin. (TOS - My Brother's Keeper novel: Enterprise)

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