Dr. Roger Korby Strange New Worlds is from TOS "What Are Little Girls Made Of?" | @StarTrekOfficial

In 2261, Korby arrived on the USS Enterprise with Chapel for Federation Day at the end of her fellowship and they surprised the crew with the revelation that they were now dating. When the crew fell under the spell of Trelane, Spock and Korby managed to break out of it, but were unable to free the rest of the crew. Trelane threatened the crew to get Spock to go through with marrying Chapel, even transforming Korby into a dog, but was forced to relent when his father showed up. When Spock questioned Trelane's motivations, he explained that he had seen Korby digging in the dirt on "the old homeworld." Trelane had found how handsome, smart and perfect Korby was annoying which Spock admitted made perfect sense. (SNW: "Wedding Bell Blues")

A few months later, Korby and Nurse Chapel conducted excavations on Vadia IX and found evidence that the M'Kroon were the direct descents of an ancient, highly advanced civilization capable of traveling across multiple galaxies, and who claimed to have discovered immortality. The M'Kroonian alphabet shared similarities with an older language found on distant worlds such as Polaris XII and Praetorian. (SNW: "Through the Lens of Time")

Christine Chapel became Korby's fiancée. (TOS: "What Are Little Girls Made Of?")

Exo III
Later in 2261, Korby led an expedition to Exo III, where he was badly injured by exposure to the planet's bitterly cold temperatures. He lived long enough to discover an underground ancient android civilization there. Using their techniques, he constructed an android body into which he transferred his consciousness. His ruined corporeal form expired that year, but his consciousness survived for over five more years as an android. (TOS: "What Are Little Girls Made Of?")