Personality and backstory are nice and all, but they're ultimately molded by other, more important things. They're the wrapper around the central core of a character: Their motivation. You can write a backstory devoid of any connection to the plot and leave the character a completely pointless, stupid thing that can be replaced by pretty much anything that has ever existed. Like NPC's who tell you their life's story in RPG's, which you don't give a shit about, because you just want them to sell you their baguette of +4 frenchness already so you can get back to the actual plot. Oh, and the personality won't make any sense unless it reflects a character's wants and needs, which--unless you select them randomly from a fucking jar--you can derive from...
... The motivation of the character to resolve the conflict of the story! The backstory can
explain why a character got their motivation, and a personality can show how their motivation has shaped them as a person, but neither of these things are central to the character: They're expressions
of the central motif of the character.
After all, pretty sure we've all read a character who has had like, a three thousand word history that was so fucking painful to read that everyone curled up into a ball and started suffering episodes of PTSD to make it stop. Or a personality field that was composed of traits so random and often with such violently opposing antonyms that the only way to explain their personality is with the words "schizophrenic bipolar escapee from the mental ward."
So...
Yeah. Motivation. If a character does not possess a motivation to resolve the conflict in the plot, then they have no reason to be there... And it will become apparent very quickly in how disinterested they will naturally be in actually resolving the plot.