His timeline was destroyed by Dr. Terminal Gaster. But he holds no anger towards him Dr. Terminal started to make him a test subject along with destroying his universe, his brother was gonna kill him. Testyfell is a pretty chill guy who likes to smoke and loves to show of his golden ribcage although, he is Underfell sans he's nice guy but since he came from opposite of Undertale, Underfell, he will act rude at times. His emotions can be controlled by Dr. Terminal Gaster with a click of a button but that never happens.
He likes to smoke, and he likes to eat Magic chocolate. He shown to like Testyswap, unlike Underfell Sans' another side who hates Underswap Sans.
Having strong hatred towards his Brother, Papyrus, he somewhat likes to beat the HELLA outta him.
He dislikes his own Au, and rejection. He was very loyal to Dr. Terminal, he hates him being mad. since it would scare him.
If you want to learn more about him go to ask-dr-terminal-gaster the maker of this is Kaci Miller/Toy Monnie