You know the rubber sheet crisscrossed by grid lines that Einstein used to demonstrate the effects of mass on gravity by dropping heavy balls onto? The heavier the ball, the deeper it plunged into the sheet, distorting all around it? Good characters should be like those bowling balls. The rubber sheet is your narrative. The thrust of your plot. A well written character distorts its passage through their unique outlook, strength of personality, and the actions they consequently take.
Think of a fictional character with real heft. Someone inimitable and striking like Granny Weatherwax. Any tale in which she were faithfully inserted would be completely changed by her presence. Add her to the Fellowship of the Ring, Rand al'Thor's party as they fled Two Rivers, have her meet Senlin as he ascends his tower - any and all tales would be changed by her addition. You cannot imagine it being otherwise.
The same goes for any character of true quality. Their insertion into a plot would change it. Whether they are noble or cowardly, forthright or conniving, a doer or a victim, they impact the tale. Subtracting or adding them makes for irrevocable change.
Why harp on this? Because if your character does not have a similar effect on your own story then they are not pulling their weight. Generic heroes suffer from having no gravitational weight. They drift on the tides of plot, doing as they are bidden, biddable and in turn forgettable. It's why Han Solo's are more memorable than Luke Skywalkers, why side kicks are usually more popular than the hero themselves: freed from the strictures of being the everyman hero, the sidekick or lancer can burst with character, prove more interesting, and interact with the plot in compelling ways.
And how do you turn your character into a bowling ball? By giving them agency. They act and are not acted upon. They instigate and do not wait. Whether with swords, words, wisdom or inquiries, they move forward. And in doing so, change the plot in a manner uniquely their own.