When First Officer Mike Holland is promoted to captain of his own ship, he is replaced by Nathaniel (Nate) Parks, a tall, wiry Scandinavian, and an old friend of the captain. My idea of an actor to play Parks is Viggo Mortensen.
Captain Darrow asks for him especially, as it is an emergency and there is no one available. He knew that Parks was available because a damaged shoulder made him unfit for active duty.
His shoulder was broken and dislocated, and although they repaired it, they were not able to re-grow all the damaged tissue, and there was a weakness in the ligaments, which meant it dislocated very easily. He puts his shoulder out again about once a year because he puts the safety of others before himself. Working on Earth, he could get treatment immediately, but if he went into space and was injured, he would be a liability to his ship until they were able to get him to a proper medical facility. He was also more likely to be injured on a PACT patrol than working in Security, where his job was more managerial.
He spent many years in Security, where he was frustrated. He got to meet a lot of other species, but he longed to be out in space. He met his wife Anna in PACT training, but she has an earthbound job, so is able to set up home and raise their two children, Lars and Olga, in relative stability.
Anna found it hard in the early days when he was away on missions, but he managed to have time at home in between. When he injured his shoulder and was given the job in Security on Earth, she was actually delighted, because it meant he was home with her and the children and they were able to build a strong relationship.
But she knows how frustrated he was, and how much he loved space. So she is happy for him to have a chance to serve a mission again when Darrow asks for him, even though there's the worry about his shoulder.
Parks is a stickler for protocol, and always refers to Darrow as 'Captain' in public, although they're good friends. He has a strong belief in discipline and decides the only way to cope with the changes in the crew, extra crew, and a dangerous mission, is to be strict. This doesn't go down well with the crew, who were used to Holland's easy-going manner. But he soon wins them round. He is mature and responsible, but likes a laugh and messing about. Life and soul of the party, given half a chance.