HISTORY- !!!Contains Spoilers!!! -

We learn over a period of time why Nick is as strange as he is, and why he roams the desert with Vash.

Nick's parents where killed when he was very young, and he was sent to an orphanage outside December city. He stayed in the orphanage until he was seven, when he killed the murderer of his parent's by shooting him. After leaving the orphanage, he meets Chapel the Evergreen, who is a Gung-Ho Gun. Chapel teaches Nick how to be a gunfighter, and leads him to become a Gung-Ho Gun himself.

When he is old enough, Nick is assigned to find Vash the Stampede and bring him alive to Knives (Vash's evil twin). He comes across Vash seemingly by accident in the middle of the desert. Vash spots him from far off, and saves his life.

Over time he and Vash develop a friendship, and he tags along with Vash just about everywhere. Whether this was his friendship or him being on a mission, it's hard to tell. He also bonds with Milly, who he seems to like from the first time he is seen in the anime.

As he stayed with Vash and the gang, he began to understand Vash's philosophy of Love and Peace. He no longer aimed to kill, just to stop. However, when the Gung-Ho Guns cross his path, he does not hesitate to kill.

For example, when he kills Zazie the Beast by shooting him in the head, Vash is dumbfounded. He can't believe Wolfwood would do that. Milly is also amazed and ashamed of Nick. He leaves for a short time after this only to return later when things cool down.

Finally, Wolfwood is forced into a duel with his mentor after rejoining Vash and company. He fights Chapel but does not kill him. Instead he walks away, but as he turns to leave, Legato takes control of Chapel's body and has him shoot Wolfwood. Nick then stumbles into a church, mortally wounded, and remember's his life. He thinks of Vash and Milly, then dies.

Vash comes across his body in the church, and takes his cross to avenge his death. Milly also carries the cross for some time, out of love for Nick. His cross is used as a fourth weapon with Vash, and it saves his life when he uses it in a duel with Knives.

So, the only real difference between the anime and the manga (in terms of Wolfwood), is that in the manga, Wolfwood is in fact Chapel the Evergreen. Chapel was made up as a seperate character in the anime for some strange reason. It's most likely because as the series drew to an end, they wanted a fitting and dramatic death for Wolfwood. I think they acheived that. Though I would have prefered he stay alive like in the manga.