Dr. Nathan Santana
Background: Doctor Nathan Santana was originally a civilian medical researcher aboard the Iasoan. In Episode 15 - "Before the Storm", it was revealed that 12 years earlier Santana had served as a Marine field medic deployed on Canceron. No details were given regarding his discharge. In Episode 16 - "We the People", Santana petitions then-Commander Benito Rodrigues to be commissioned again as a medic (Day 75). Rodrigues instead offers him a commission as a Captain and assigns him as a Doctor aboard the Libra.
After the Chiron was recovered, Santana was among those who volunteered to serve as the vessel's commanding officer. He was granted that assignment circa Day 95.