I don't think your approach to character building is effective. You might make it work; different things work for different people, but I think (because you asked this question) this approach isn't even working for you.
All of the following, of course, is my opinion and my approach; so I will dispense with caveats and waffling for the sake of brevity.
An idea of "stoicism" is a result, it is not where you start. Characters are formed by their upbringing. In real life, this consists of thousands of events; in fiction we focus on a few unique "catalyst" events for our character. Now many such events depends on the "depth" of the character, how many scenes they may be involved in.
If it is just one, you may not need any; you can use a "walk-on" stranger with any personality you like to portray the shopkeeper, a waiter or random customers at the bank.
The more scenes they are in, the more justification you need: Your main character needs a backstory.
But a backstory is a story, the reasons why they are who they are; the formative events that turned the blank slate of an infant into the character before us.
Why are they courageous, or cowardly? Why are they promiscuous, or chaste?
A character is not a list of random traits you assign. They are the result of shaping forces in their lives, their hardships, their losses, the praise they received or were denied, the emotions that were encouraged or ridiculed, their attempts to excel that succeeded or utterly failed. The behaviors that were beaten out of them, and the behaviors that brought them praise and delight.
A friend of mine once observed, "Parents can dislike other people's children and love their own, because they have beaten out of their own children all the behaviors they don't like."
So if you want "stoicism", you need to come up with the how and why this person grew up to be stoic.
If you want a ruthless assassin on the side of justice: What key events in her life brought her to this state?
Don't just assign her the trait; what made her this way? Obviously it has to be something that doesn't happen to most of us growing up, but somehow, her past lets her pull the trigger when she is personally certain the world would be better off with a person dead, without being emotionally bothered by this at all. Perhaps she uses seduction and sex as well, but that may need a justification in her life as well: She was a virgin at some point, so what happened, and when, that made sex become a tool for her? Specifically, what is the scene that made her that way?
If you have a woman with a husband that openly cheats on her and brags about it, what in her past makes her tolerate this?
I may begin with an assigned trait: The ruthless assassin. But I don't stop there, I do not let myself use traits unless I can imagine a plausible set of turning points in a life that led to it. And then the other traits, consistent with those same turning points, become evident, for a plausible character with multiple traits.
I don't afford myself an unlimited number of these "shaping" scenes, maybe three or four for a main character, and one or two for secondary characters. Characters that have only one or two appearances in a story have no backstory at all; like all strangers we meet, they are who they are.
So my characters seem realistic and complex, but they aren't really. They are always consistent with their life-shaping events, which inform their beliefs and actions.
Usually the story is, in fact, about another such life-shaping event, that will change them (and perhaps some secondary characters) yet again.
My final advice, on stories in general, is we must learn to truly hurt our characters. These life-defining events are often traumatic, dealing with death, injury, gross misjustice, violations and loss. Although love and kindness can be a shaping force in our lives, the most awful events are typically those with the most negative emotion. Somebody they loved died. Or they themselves were victimized and helpless.
Stories are about heroes being knocked down but getting back up, again and again, against daunting odds but refusing to give up. That's what makes them a hero.