Dr. Sans is the Royal scientist of the underground. He has created many inventions that appear in the world, including the core with the help of Dr. Alphys. While Alphys is more focused on the mechanical aspect of science, Sans works with quantum physics. He is also partly the creator of Mettaton. He has a very high authority, allowing him to access many places and, seeing as he built most of the Undergrounds technology, can control anything he doesn't have authority to use. He acts as a guide an partner, helping you and being near a lot of the time. This causes him to be present during the meeting of many main characters. Unlike Undertale, Sans, while still laid back, is a lot more motivated.
Sans, instead of wearing a jacket, wears an off-white lab coat with a rim around the edges, slight fur around the top, and a pocket to carry pens. He has a faded blue shirt underneath with the star man symbol on it, though the closed lab coat makes it unseeable. He still wears the same pants as before, although he now wears proper shoes, which are dark blue. He has a robotic arm thanks to the incident at the core, that can only be seen when shrugging. Thanks to the same incident, he also has one eye that is not functional, and has screws over it from repairs. During the genocide route, his sleeve is torn, showing his robotic arm, and his lab coat it open. His skull also has cracks in it and his broken eye has a glowing white cross, similar to Mettaton Neo's.
Sans, although still laid back, is far more motivated than in Undertale. His inventions have shaped the Underground, and he does his best to keep you alive. If worst ever comes to worst, he's willing to fight to save his friends. During the genocide route, he is harder than his Undertale counterpart, as he is much more motivated and is actually giving it his all.
Sans is not as familiar with Toriel as in Undertale, mostly knowing about her from Papyrus's descriptions of her. He does, however, decide to uphold Toriel's promise, since Papyrus couldn't.
Sans's relationships with Papyrus is the exact same as in Undertale, though he doesn't take care of him as much since Papyrus is less naive and actually has a job.
Alphys is Sans's assistant. Though she thinks he is better than him, Sans respects Alphys for her technological accomplishments that he could never do. He has become good friends with Alphys, and since he is involved in the events of the true lab, is always there to back Alphys up.
Sans guides the player throughout the game. He is more often seen and uses his authority to help and diffuse situations. Aside from that, he is similar to how he is in Undertale.
Sans, similar to Undertale, knows most people around, especially in Snowdin. He also has relationships with both Undyne and Asgore, but nothing too extreme to be notable.