This is a video game exclusive trope, where a certain character is only heard in recordings you find or read about in letters. They can go through whole character arcs, and it's this that makes them not the Ghost or the Voice; if all artifacts related to them were viewed chronologically, it'd basically be a story within a story.
This certainly needs a better description and name but I think it deserves a
- Bioshock Infinite has Preston E Downs, whose life never intersects with Booker's but has an arc where he goes from racist Manhunter to protector of a native American child.
- The Last of Us has Ish, leader of a now abandoned encampment. His letters detail the rise and fall of his band.