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Each hirable NPC has 7 skills . They're somewhat randomly generated and only the bold ones and each skill has 5 levels of competency (the more stars, the better). Numbers of stars are somewhat randomly generated in that I don't think it is possible to get NPCs with really low numbers of stars any more. 

Only the skills marked in bold font are relevant to the performance of the NPC in question (their Primary Skills). In most cases, each NPC will have three Primary Skills but Engineers just have one. Marine Officers are somewhat unique in that only they have a skill that improves with use, "Boarding Experience".

Info on exactly what for and how they In Rebirth 3.00, Virtual Seminars were introduced to the game, which allow us to add a star to a Primary Skill of our hired NPC. They are available via Smalltalk from other NPCs and this feature is described in more detail in the official guide to 3.00 here and on this wiki here. Virtual Seminars cannot be used to increase a Marine Officer's Boarding Experience but since Virtual Seminars take some effort to acquire, I would suggest that would be a waste anyway.

Info on exactly how the skill values are are used is sketchy but CBJ has revealed a few titbits.:

"The relevance of each skill depends on the type of NPC. For example for a pilot there's a ratio of 5:3:2 for navigation, combat and morale, and for an architect it's 3.5:3.5:3 engineering, management and navigation. For an engineer the only relevant skill is, as you might expect, engineering."

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Further info on Marine Officers from w.evans:

 "Boarding Experience 15, Leadership 3, Morale 2. Current as of X:R 3.10. This didn't come from the devs, though, if that's important."