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Many stories have characters who performs offscreen heroics or have their own adventures (as TVTropes calls it, The Greatest Story Never Told). Example off the top of my head is Glynn Stewart's ONS...
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Many stories have characters who performs offscreen heroics or have their own adventures (as TVTropes calls it, The Greatest Story Never Told). Example off the top of my head is Glynn Stewart's ONSET series: one of the good guys is a demon named Ix. At some point between books he went off and did _something_ that allowed him to overcome the instinctual need to obey a higher-level demon, so he no longer fears going up against one. At no point is it explained what he did or how he did it. In the final book, when the main character, David, is facing down the entity that's the major threat to the world during a demon invasion, Ix is off on his own. He only shows up at the end to reveal that he confronted and defeated the main demon general and "ate" him (somehow, it's never stated if that's literal or metaphysical), making him the highest-ranking demon around so he was able to command the demon army to stand down. In both cases it works because it's not a main viewpoint character. Ix is an ally, not the center of the story, so it's accepted by the reader that he can be off and doing his own thing while the story is focused on David, the central and main character. Another, more well known example where it is a main character. In the original release of _The Return of the King_, Aragorn, Gimli, and Legolas are last shown on the Path of the Dead, confronting the Army of the Dead. The next time they show up is when the Corsair ships arrive at Osgiliath, supposedly bringing reinforcements for Sauron's army, and the three come charging off the ships leading the Army of the Dead to victory over Sauron's forces in the Battle of the Pelennor Fields and relieving Gondor. In that original release, they never show Aragorn intercepting the Corsairs and seizing their ships. He just shows up. The audience simply nods. "He's freaking Aragorn. Of course he pulls something like that off." I can cite numerous examples where a character does something like that, but the successful ones are those where it's a secondary character and what they do is in support of the main character's plot, or it's a character who is established to be one where pulling off something like that is pretty much par for the course. "Well of course she did that. She's [name]. That's what she does."