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.
Papyrus would be a great addition. His relationship with Sans could take on a whole new dynamic when Sans becomes human. Papyrus might be overprotective at first, not sure how to deal with his now - human brother. He could also be a source of comic relief as he tries to understand human behaviors.
The other Sans could also be very interesting. If it's a Sans from a more light - hearted universe, his reactions to Horrortale Sans would be a sight to behold. He might be shocked, intrigued, or even try to help Horrortale Sans in some way. His attempts to understand Horrortale Sans's world and bring some of his own positive energy into the situation could create a lot of engaging storylines.