 If you liked
Trigun
|
 ...then you might like
Ansatsu Kyoushitsu
|
Both series revolve around a surprisingly nice living natural disaster, both of whom have enormous bounties on their heads, and take a pacifist approach despite how easily they could kill anyone and everyone who stands in their way.
Both characters caused widespread disaster in their backstory for reasons that are kept hidden. Their motivation revolves around a woman in their backstory whose significance is kept vague.
They also have a brother with similar abilities, who they are enemies with.