Okay, I've talked about walk-on and background characters, so I guess it's time to take a look at 'secondary' characters. These would be the friend or sibling that the main character relies on for advice or help when he/she has a problem to solve. You need to know quite a bit of their background, because their history will shape the advice they give, the way they react. A story can have several of these, one or possibly none.
If you look at the old, original Star Trek shows, it is generally agreed that there was a triad of 'main' characters; Captain Kirk, Spock, and Dr McCoy. Secondary characters were Lt Uhura, Lt Sulu, Lt Cmdr Scott and Ensign Chekov. Some people consider Nurse Chapel and Yeoman Rand secondary characters, also. My feeling is that, in one or two stories they were secondary characters, but for the most part, they were background characters, along with Kyle and Riley. And of course, with a crew that large, there were any number of background and walk-on characters.
Since Mac's story (at least, the first part of it) also occurs on a spaceship with a crew in the hundreds, there are several characters who are definitely secondary, and quite a few who fluctuate from being secondary to a background character, depending on the scene. Consequently, I have to have a pretty good idea who all of these characters are, from her best friend (trying so desperately to keep her out of trouble), her roommate, the officer put in charge of re-training her, once it's decided she's in the wrong field, and even the man who wins her heart (an event that nearly destroys them both). I even had to do considerable 'research' into the culture and biology of the alien officer charged with seeing that she was treated fairly, since the captain kept getting conflicting reports of her capabilities.
Knowing your characters can be time consuming. But it's worth the effort when you create 'real' people, and not cardboard puppets. Not only do your stories come alive, but you have a whole host of new friends!