It could also be an organization or a group named Epic. They might be involved in the business of storytelling or literature, and they are the ones who read the rest of the story. For instance, they could be a library group that reads out the rest of a story during a special event.
The person behind Comic Sans is Vincent Connare. He developed it while working at Microsoft in the 1990s. It was initially intended for use in a children's software interface.
Alphys could be another common character. As the royal scientist, she might be called upon to figure out what's wrong with Sans. She could bring her scientific knowledge to the table, trying to diagnose and find a cure. Her own insecurities might play a role here too, as she might be worried about failing to help Sans.
He might also interact with Rias. Rias is a powerful devil and a leader. Sans could approach her with his laid - back attitude. She might be initially suspicious of him but later come to respect his abilities. They could have some strategic discussions about dealing with common enemies in the High School DxD world. And Sans could probably crack some jokes that would either amuse or baffle Rias.
Another possible main character could be Frisk. Since Frisk has a connection with Sans, they would be involved in Sans' story when she becomes a girl. Frisk could be the one who first discovers Sans' transformation and helps her come to terms with it, or they could be on an adventure together with Sans in her new form.