I know this sort of seems more about plot-development based on the title, but the reasoning is much more geared towards MC's personal character development. I'm currently writing a sci-fi series about a character who operates what equates to shipboard nuclear reactors. Being that this job is incredibly boring to read about, he's going to branch off and become a much more action-involved character, with this process starting while he is an operator, learning the new skills as a hobby, being taught by a friend who is essentially Special Forces. The friend proves to be a more level-headed individual and, seeing as he's teaching, he takes on a mentor-role as well for the main character, driving his character development. Really bad stuff happens. MC is saved by Mentor-Buddy. MC has moment of success and helps save surviving party, but everyone's still all super-depressed because of the really bad stuff that happened. MC gets training to be superawesome by military, with mentor-buddy's chain of command pulling some strings to help him out and he gets what are essentially technological superpowers that all special forces dudes have. One thing I need to work out: I want them to stay together for at least a good portion of the following story, but military logistics being what it is, this is unlikely. I can explain this away, though. Killing him off would add drama. Statistically speaking, it's combat, so people will die, and it will cause MC pain. He doesn't need more pain, though. But if I do, it will drastically alter the MC's state of mind further into the hole I want to drive it into - "the worst enemy he has to overcome is himself" - so that he can have a moment of actualization and victory in the end, where he manages to recover from a pit of anguish and hatred for those who have wronged him, and proves to be the better man, forgive himself, yadda yadda. What I do need are more solid characters to work with, though. I have plans for a couple, but it's going to be hard to have many due to the transient nature of friendships in a military lifestyle. This one mentor-character is one of the few MC can actually have. Do I kill him off to drive the plot? Keep him around to kill him later, i.e. potential sequel? Or just not kill him at all, leaving him to go off on his own adventures, eventually?