(1) Physical characteristics.
This is anything that will help you visualize your character moving in space. 
These can include: age, height, weight (or body type), eye color, hair color, skin color, scars, tattoos, left/right-handedness, verbal tics, inheritable conditions (color-blindness, diabetes, high blood pressure)

(2) Social markers.
These "markers" are social constructs, and carry meanings that are particular to each time period and location.
They include: race/ethnicity, gender, nationality, class, sexual orientation, education, dis/ability, religion. As a sidebar, I recently listened to a talk by K. Tempest Bradford on intersectionality, and she makes the point that social markers such as race and gender and disability inflect each other. This may be something to consider as you’re developing your MCs or SCs. I have a short essay elsewhere under the FOR AUTHOR tab about this, if you’d like to know more.

(3) Personality traits 
I’d say pick 5 for now, when you're starting. Generous, selfish, anxious, ambitious, sly, naïve, fussy, an insomniac, impatient, insightful, empathetic, shy … 

(4) Shorthands
In our Western culture (again, these are era and place specific) these "shorthands" gesture to what is almost a small plot-line, or a set of traits that we might expect.  
Examples are: divorced seven times, only child, orphan, ex-Marine, Depression-survivor, cat-lover

1: Secondary characters need to further the plot or resist the MC’s plot arc. The acid test is if you can take this secondary character out and nothing in your plot changes, then they don’t belong. It’s very sad, I know—but you have to kick them out. Tell them it’s not personal, that you like them, and they can go in a different book! Maybe they deserve one of their very own! 
 
2: Eliminate redundancies. You will also find some Secondary Characters can be combined. For example, in LADY, I had two sidekicks to Tom Flynn, one an ally and one a foil. I cut it to one, Jeremy, who could serve as both.
 
3: Don’t introduce your secondary characters all at once. It’s way too confusing. Space them out. Each time you introduce one, put [NSC] (New Secondary Character) in brackets in your manuscript. As you get toward the middle-ish, or even the end of writing your draft, do a SEARCH for them. Are there too many, too close together?
 
4: Help your reader keep them straight. A few good ways
(1) Distinguish the voices. In Stef Penney’s TENDERNESS OF WOLVES, she has four or five different narrators. And the voices are so different you don’t even need to be told who’s speaking. This difference in voices will often evolve naturally if you’ve done your off-stage work.
(2) Associate characters with a particular hobby or place. Someone who always is fiddling with his stamp collection or playing her piano. Someone who goes to the same bar every day after work.
(3) If it’s been a while since we’ve since the SC, remind the reader with a bit of dialog or internalization of who she is.
(4) Make sure your character names don’t sound too similar. Blackwell and Boulter and Bingley and Burns … change it up.

I learned more about this complex topic recently, listening to a talk by K. Tempest Bradford who explained that the origin of this idea was the lawyer and feminist scholar Kimberle Crenshaw’s 1989 essay, Demarginalizing the Intersection of Race and Sex.
​Crenshaw's argument is theorized and complex, but the main point, for me, is that social markers such as race, gender, sexual orientation, dis/ability, class, education, nationality, and religion inflect each other. In thinking about identity, and the way these markers affect people's lives, they cannot be considered separately. For example, as scientific studies have shown, a chronic pain sufferer (that is, someone with a disability) who identifies as Black may have a harder time obtaining good medical care and pain meds than someone who is white.
I’ve been thinking about intersectionality more these days, and the way my fictional characters' traits and social markers inflect each other and shape their actions and reactions in my stories.