I'm writing about hidden narratives in games, and Sans... is actually a way more interesting character than he's been made out to be by the internet. Besides just taking what the game shows you through both the Pacifist and Genocide routes... there's also
this.
Basically, Sans has this very nebulous and intriguing backstory involving science and other characters never seen in Undertale. What his actual backstory is, no one knows. It's commonly believed to be associated with W.D. Gaster, the secret character that has yet to be fully explained. There's a lot of other small information that can be strung together to determine that there's much more going on with Sans than meets the eye, but there's no conclusive answer to what it could be.
These are the kind of Undertale discussions I saw back in 2015 when I first played the game, and it's why I have a very positive view of the game, and it's really sad that so much of the depth and craft that's there is ignored because it's easier to meme about the characters, I guess.
So I'm using that as an example of a hidden narrative in a game that fleshes out a character more than what is seen in the main plot. "Hidden narrative" meaning something that is optional and not entirely obvious to the player. That is, they can complete the game and never observe it.
Not this time.