Perhaps you think it's more interesting than the main story, or perhaps you just think it's interesting enough to be the foreground and woefully inadequately treated. . . .
The last can be interesting. Because it can mean that it is not only not, in fact, an intact story. It can mean that it's not (in the original story) capable of being an intact story. The structure is not merely incomplete, it's flawed. Perhaps the character is set up in a pickle such there is really no way out because it was not the story and didn't have to be resolved.
This calls for structure changes deep in the idea. Perhaps from the very beginning. Which, at least, helps file off the serial numbers.