Yukito Nogi, a high-school student living by himself in the city, was out walking one night when a strange object fell from the sky... crashing straight into him. This object turns out to be a cosmic force, its power so great that it splits Yukito into three different entities, each composed of one part of his psyche. It also happens to give each part enormous psychic powers!

However, he's only just started arguing with himself, when three different factions converge on him all at once. A trio of aliens, led by the self-aggrandizing scientist Olga Mengele, want to steal Yukito's power for themselves. Yuki and Yuka, a pair of sister demon hunters, believe Yukito to be a demon that must be sealed. And of course Hiromi Togo -- alias Trigun, defender of justice -- wants to use these events to prove herself as a superhero.

[From Mobygames]