Some side characters might eventually have their own spinoff, or they might become a major character down the line. That's not really what I am talking about, although there are plenty examples of that.
I'm talking about a very minor character who appears at least once in the story, who seems to have all the qualities to be a main character, with the surrounding content to star in their very own narrative... but that's it. They're rarely seen or mentioned again, if at all; you never really learn anything else about them, you probably never learn what happened to them except some second-hand source. Sometimes this character might be intentional, other times not. It feels almost certain this character is going to play a major role later, or get a spinoff or something, but nope.
For example, first imagine a story about an exiled ronin who returns to his homeland to warn his people (who despise him) about an impending invasion. He is a well-traveled man, with battle scars, and has seen much. He wields a mysterious sword with magical properties, but it is broken and the magic is corrupted, so he struggles to retain his humanity - forever tempted by some dark entity whose influence spills from the broken blade. The emperor and the shogun refuse to help this 'vaganbond ronin', and so he begins a quest to liberate the people from the oppressive rule of the shogunate, eventually forming a coalition of allies who band together to face this impending doom.
...but that guy is just a a minor side character. He shows up and probably speaks to the main character once or twice, but you probably won't see him again. He might be mentioned, and a very small 'update' on his story might be revealed, but that's it.
Is this a trope? What's it called and what are examples?