Zen [Sans] is a neurodivergent skeleton who suffers from Intermittent explosive disorder and sadly killed his brother during one of his outbursts, as soon as he realised Zye [Papyrus] was dusting from his action, he panicked greatly, searching for anything to help his brother, but by the time he got back with a sour bun [healing item] Zye was already dead.

He got so angry with himself that he had another outburst and punches a wall, which in that moment he noticed his level was higher, from accidently killing his brother. He felt distraught and disgust from doing it, it swore to kill every single monster to gather up enough lv to power a determination machine to reset the timeline.

[if i turned it into a game, you'd play as Zen, it was begin where Zen was having his outburst and killed Zye]
Zen has only 2 friends, Toriel and flowey, since he knows they'll have an importance.. because how many times has he "accidently" done this?