Lore explanation for things in the drawing:
Torn and tattered clothes
Sans has been in thousands of fights by this point, and he's had a great number of close calls and tough fights. He's also just worn them all the time for what has been years at this point. His scarf is the notable exception, and he keeps it pristine as to not disgrace his brother's memory.
The souls inside his silhouette
Sans has been traveling through timelines, chasing after the SOUL after an error during a reset caused him to remember every past timeline he had forgotten. He has absorbed the SOUL in hundreds of different timelines, trying to cut them off every single vessel they have across different timelines, effectively sealing them off from the world for good.
The various snippets of Undertale screenshots (and the Deltarune one)
Sans has traveled through hundreds of different timelines as I have said, and across these many timelines, Sans' is not the only aberration. Sans has met many other unique people in certain, special timelines.
Sharp bone
Looked cool idk
