When Vash's bus runs across Wolfwood's motorcycle in the middle of the desert. He proves to be a valuable ally although his and Vash's philosophies often conflict, Wolfwood being a firm believer in martialism while Vash is a pacifist. By the end of the series, he agrees with Vash's way of thinking even though it costs him dearly. Wolfwood's purpose is to raise money for the children in the orphanage he runs.
The giant cross that he carries comes in two versions. Version 1 holds 12 pistols (across arms of the cross). Version 2 houses a giant machine gun (long part of the cross) and finally, a rocket launcher in the top, while the ammunition is stored in the arms of the cross(an alternate opinion is simply that one of the horizontal arms holds ammo for the machine gun and rocket launcher while the other holds six pistols). The trigger for the Version 2 is a skull shaped symbol in the centre of the cross. As can be expected, the weapon/case is very heavy, and only Wolfwood, Milly, and Vash seem to have no trouble holding it (though Vash once comments on its weight). The weapon is not actually named in the anime, but it is commonly referred to as Punisher, which comes from the manga. Wolfwood states that it's heavy "with God's mercy," a fitting statement from the cynical Wolfwood.
He is modeled on Tortoise Matsumoto, the lead singer of the Japanese rock band Ulfuls. The name Wolfwood (ウルフウッド urufuddo) is a corruption of Ulfuls (ウルフルズ urufuruzu), which is in turn a corruption of the English word soulful (ソウルフル sōrufuru).
Nightow is frequently asked in interviews and at convention panels what the "D." in Wolfwood's name stands for. His responses have varied each time, ranging from "Dangerous" to "Dokonokuminomonjawaresumakinishiteshizumetarokakora." The latter is actually stereotypical yakuza slang, and would be written out as "どこの組の者じゃ、われ！？ 簀巻きにして沈めたろか、こら！", which roughly means "What the hell family do you think you're from!? I'm gonna tie you up in a reed mat and dunk ya!". The common theme in these various expansions of the single letter "D" is one of destructive force, anger, or violence, which are traits Nightow has modeled the character after.
In the original Japanese version, Wolfwood speaks in a Kansai Dialect. Nightow has said of this: "It's an English-speaking world, so he's not actually speaking Kansai dialect. Think of it as indicating some accent."
Wolfwood is actually the student of the Gung-Ho Gun 'Chapel the Evergreen'. It is strongly hinted that Wolfwood would have inherited the title once his mentor completed his service to Knives. Wolfwood was taken in by Chapel as a child after escaping an abusive home situation. Nicholas was raised by his alcoholic uncle whom he eventually shot dead; this survival instinct apparently appealed to Chapel, who raised and trained Wolfwood from early adolescence. Although he is a professional killer, Chapel seems to sincerely care for his ward.
Spoiler Warning for Manga Wolfwood: Wolfwood was raised in an orphanage before being inducted into the Eye of Michael, an organization of assassins founded by a plant worshipper. The Eye of Michael worships Knives as a god and provides him with contract killers. Wolfwood's teacher and master was Chapel; a man much different from his anime incarnation. During his training, Wolfwood had been physically enhanced to superhuman status, including regenerative abilities that are triggered by consuming vials of a special drug. This also speeds up his metabolism resulting in him looking much older than he really is. In the manga, Wolfwood is somewhere around 14-16 years old.
Shortly before the series begins, Wolfwood shoots his mentor, leaving Chapel without the use of his legs and taking his Master's identity so he can join the Gung-Ho Guns in his place. After the fifth moon incident, he attempts to kill Knives but is overcome by fear and is unable to move. Instead of punishing him, Knives gives Wolfwood the task of protecting his brother until Knives is able to see him again. This causes many of the other Gung-Ho Guns to assume that Wolfwood is a traitor although his actions do not directly contradict Knives' orders until Volume 8 of the manga, when Wolfwood breaks into the ark in order to rescue Vash.
The manga version of his weapon is part of a special series of guns named "Punisher," and the highest honor in the Eye of Michael is to receive one, since the Punisher is considered to be the strongest personal weapon the Eye of Michael possesses. His teacher Chapel tells him that his Punisher is the tenth to have been built in the 133 years of history of the Eye of Michael. The manga Punisher differs from its anime counterpart slightly in coloration, and in that the two side arms of the cross are used to hold the ammunition for the main machine gun rather than additional pistols. His "true" Eye of Michael title is Nicholas the Punisher.
(Source: Answers.com, Wikipedia, Nicholas D. Wolfwood)