Underfell!Gaster headcanons
Gaster, the royal scientist, always puts on a polished, gentlemanly front when he’s around other people. Sans jokingly calls this “kissing up to someone’s ass,” and Gaster knows exactly what he’s doing, even if he never names it.
His father spent years training him to adopt that persona: speak respectfully, act like a true gentleman, and hide any irritation behind a courteous smile.
Without that training Gaster would be far too obvious when annoyed—sometimes even daring to talk back to his superiors. His parents saw the change as a sign of maturity: he stopped openly arguing with his superiors or flaunting his “more accurate” theories in public.
He still thinks the same thoughts, but now he conceals them, slipping into his true self for a split‑second whenever the person he’s speaking to looks away, before snapping back to his practiced smile.
Most people enjoy teasing him about his double act, but the joke can turn dark quickly. One day, while Gaster was working in his lab, a lower‑class worker, a junior assistant offered a 'suggestion' for improving one of Gaster’s experiments. Gaster didn’t take the suggestion politely; he marked the worker’s face permanently, a stark reminder that he never tolerates input from those he deems beneath him.
The worst fate for anyone who draws Gaster’s ire is to become the object of his obsessive fixation. When something captures his interest, he hyper‑fixates on it.
If that something is a person, his obsession becomes all‑consuming. Even a work‑aholic like Gaster can become terrifyingly relentless, turning a single slight into a personal vendetta that he pursues without mercy.