Esaak was a Valakian physician and clinic director who lived on Valakis during the mid-22nd century.
In 2151, he was one of many physicians trying to cure an epidemic that had hit all the Valakians. The planet had two humanoid lifeforms, the Valakians, and the Menk, but only the Valakians were affected.
With the help of Dr. Phlox, it was realized that the epidemic was really a genetic virus. He asked Captain Archer for the schematics to their warp core so they could perfect warp drive for their species to allow them to travel out in hopes of finding someone who could cure them. Archer refused in order to let evolution take its course, a decision that was similar to the Prime Directive, which Starfleet adopted several years later.
After he called Archer to ask whether Phlox had made any progress in treating the epidemic, all they could offer was medicine to help ease the Valakians' symptoms and pain. (ENT: "Dear Doctor")
In the final draft script of "Dear Doctor", Esaak is described as "the director" of a Valakian hospital he is shown working in. Esaak was played by David A. Kimball.