Story and Characters

Vash the Stampede
A gunman wandering the desert, Vash has made something of a name for himself. The "Stampede" nick comes from the mass destruction that seems to follow him whereever he goes. Whole towns and cities have been destroyed with Vash being named as the culprit, and because of this he has a $$60,000,000,000 reward on his head(Dead or Alive T_T). However, the viewer finds out almost instantly that the damage done is usually the fault of bounty hunters, bandits, or other people seeking Vash out for the reward, or some reason or another.
Vash carries a large silver gun, has a cybernetic left arm, and also sports a bright red trenchcoat and yellow sunglasses; on the outside he appears to be an easygoing, friendly guy who can't drink without puking and tends to be a little perverted when it comes to girls ^___^. However, upon observing him in a gunfight or some other scuffle, Vash is noted for his agility and shooting skills. He almost never catches a bullet(even at close range), can knock bullets off-target with a small rock thrown from 30 or so yards, and manages stunts most humans are incapable of pulling off.
Though he seems happy and lighthearted on the outside, Vash hides quite a bit of anger, frustration and sorrow, and rarely allows it to show. He daydreams of a dark-haired girl named Lem(See Spoilers), and is uneasy over killing others(if he is forced to protect himself or others, he tries only to wound). Vash is on a personal quest, though what this entails has yet to be revealed(see Spoilers for more info). Vash has no family to speak of, but allies himself with Wolfwood, and Bernadelli Insurance girls Meryl Stryfe and Milly Thompson("Doumo!" ^_^).

Nicholas D. Wolfwood
While riding to May City on a bus across the desert, Vash spots a very dehydrated Wolfwood in the sand. Wolfwood is a priest travelling in the name of(so he says) an orphanage/church near December City. He carries a large cross wrapped in cloth, but it's revealed to be a rather elaborate, portable arsenal.
Wolf sports a black suit and sunglasses, smokes, and speaks badly-accented Kansai japanese(dialect spoken in the Kansai region of Japan, including cities like Osaka and Kyoto); he has a kindness for children and others in trouble(though one doubts his intentions when it's an attractive woman he's assisting). Aside from his cross, he carries a "portable confessional", which he offers in exchange for money or other favors(most turn it down ^^;).
Though Wolfwood insists that he's a man of the church, he has shown remarkable skill in handling guns; his past is not known when he enters the series, but without a doubt there is more to Wolf than he would have us know. He allies himself with Vash and Meryl, and in particular develops a fondness for Milly ^_^.

Meryl Stryfe and Milly Thompson
Meryl and Milly are disaster investigators sent by their company, Bernadelli Insurance Company, to check out the scene of Vash's "Crimes"(To make a long story short, the destruction that seems to follow Vash is causing the company to lose money, and Meryl and Milly are trying to prevent that). They've been ordered to keep an eye on Vash around the clock, to keep whatever could happen in check. This, of course, doesn't work out at all from the first couple of episodes on(Though Meryl is not convinced until Episode 5 that the lunatic they're following even is Vash).
Meryl Stryfe is the senior of the pair(Milly calls her 'Senpai'). She's very straight and by the book, and very intent on her job. She likes to think she's up to snuff in dealing with all kinds of people, often offering donuts and extra manners when approaching them. ^_^ But she gets flustered when things don't go as planned, and spends a lot of time beating on poor Vash(before she finally realizes that, perhaps, this incredibly strange/perverted/lucky freak isn't as bad as she believed..). She starts to soften around episode 12, and becomes one of Vash's staunchest supporters.
Milly Thompson is the youngest of 10 children, and acts it. ^_^;; She's Meryl's second-in-command, and has a very childlike, kind and patient personality. She often gives off the impression of being simpleminded, but we learn early on that she's sharper than most of the other characters( ".. My only advantage is that she doesn't realize how sharp she is.."). Milly, while being childlike, is absolutely dedicated to the ideals she learned from her family, those of justice and the all important "Do the Right Thing". She demonstrates the ability to make sacrifices on her part for the sake of others, with little to no hesitation(Wolfwood admires her greatly for this trait).
Meryl is shorter in appearance, with short, dark hair, and dresses mostly in white; she arms herself with a mini-arsenal of hidden derringers. Milly is unusually tall with long, light brown hair and a teal and tan overcoat; she carries a humungous stungun that shoots X-shaped, metal projectiles.

Kuroneko-sama/Lord Black Cat
Kuronekosama is...Well. Usually referred to as 'It' instead of a he or she, Kuronekosama is a small, green-eyed black cat that has little if any bearing on the Trigun series, yet appears at least once in each anime episode. Its formal name is "Kuroneko Kuukan" or "Black Cat Space". Many people I've talked to find its presence almost disturbing sometimes, but usually Kuro is there to add a laugh or two to what might otherwise be an overly-serious situation. ^_^

Spoilers!
Coming...