Sans Koopa looks like Morton but has Sans' face.
Sans is laid back, often sleeping on the job and taking breaks. His laziness is a combination of fatalism and apathy. He wonders if his indolence stems from his knowledge that any progress he makes will be erased when the timeline resets. Conversely, he suggests that this knowledge could be "a poor excuse for being lazy.
Though Sans is usually agreeable, he becomes eerily serious at particular moments. He is also observant; Sans reads the protagonist's expressions and can often tell when they have already done certain tasks. He enjoys making bad, skeleton-related puns, although the fact that he performs at MTT Resort hints that he may be a talented comedian. Sans may have a scientific background. Evidence includes the quantum physics book, workshop, and his relationship to Alphys.
Sans can quickly travel or teleport using what he calls "shortcuts."
In combat, he uses Gaster Blasters, devices that resemble a skull, to shoot a kind of laser beam. Sans excels in combat and fights with unique abilities that include: Karmic Retribution, changing the effect of gravity on Mario, and attacking the protagonist in the pause screen. His magic reacts to EXP; Karmic Retribution would presumably be weaker if the protagonist had little EXP.
Because of his "research," he has knowledge of timelines and the SAVE function. However, he does not retain his memories when the protagonist loads a SAVE, and he cannot use the SAVE function. His awareness of time travel makes him scrupulous, and he recognizes when the protagonist is behaving unusually. Sans reads the protagonist's expression and determines if they have experienced events before and are repeating them. He also knows how much EXP the protagonist has and comments on it when judging them.
Sans also plays the trombone, though he is only seen playing it once. Papyrus finds his "incidental music" annoying, implying that he has played many times before.
Like other characters, Sans is aware of when the game's code suffers bugs and calls the protagonist a "dirty hacker" if they achieve an impossible ending of Super Mario World.