Bill is the main antagonist in the Kill Bill movie series. He was the leader of the Deadly Viper Assassination Squad, where he was known by his code-name Snake Charmer. Bill was Beatrix Kiddo's love interest, and the father of her daughter. He was killed by Beatrix, using the Five-Point Palm Exploding Heart Technique.
Bill and his brother Budd grew up without a father (although Esteban Vihaio, a pimp, acted as a father-figure for a time). He was once a student of Pai Mei and of Hattori Hanzō, whom he betrayed. Eventually, he establishes his own assassin-for-hire organization, the Deadly Viper Assassination Squad.
It is commonly thought that Bill (not long after his betrayal of Hattori Hanzo) was also a part of the killing of both O-Ren Ishii's parents.
Deeply saddened by the apparent murder of his second-in-command, and lover, Beatrix Kiddo, Bill personally sought out her killers, only to find that she was still very much alive, trying to clear her past and start a new life with her unborn child.
Bill surfaced unannounced at Kiddo's wedding rehearsal at the Two Pines Wedding Chapel, and promised not to interfere with her new life. When Bill met Kiddo's fiancé, Kiddo covered for Bill by saying he was her father. Bill wished Kiddo the best of luck as he moved to the bride's side of the wedding chapel. At the same time, the remainder of his assassin squad moved in from outside, and massacred the entire wedding party. Bill delivered a final speech to Kiddo, after which he performed the coup de grâce by shooting her in the head. Before he fired, Kiddo tried to tell him that he was the father of her unborn child.
After the deaths of O-Ren Ishii and Vernita Green, Bill visited his brother, and warned him that Beatrix will come after him. He asked his brother if he kept up his Hanzo sword, but Budd told him he pawned it.
Having dispatched Bill's henchmen and women, Kiddo finally confronted Bill, only to discover their child was still alive and loved her father. Bill reminded Kiddo of her own violent ways, and the fact that love and violence are far from exclusive. Kiddo, who had heretofore operated as a machine-like instrument of vengeance, was shaken but ultimately undeterred. After spending the evening with a peaceful dinner and a movie with their daughter, the two had a brief sword fight. Kiddo tapped Bill in the center of his chest, using a secret "Five-Point-Palm Exploding Heart Technique" that was taught to Kiddo (and not to Bill, or anyone else) by Pai Mei. Bill understood the significance of this move, knowing that he would meet his death from an exploding heart once he took five steps from where he stood. "Well," he smiled, "how do I look?" "You look... ready." Kiddo told him. As Kiddo cried a little, Bill stepped into the darkness of his backyard and collapsed - Kiddo's vengeance collected at last.
- Handguns: Bill frequently used handguns on his targets and demonstrates a lightning-quick draw with his revolver and precise aim with his truth-serum gun.
- Swordsmanship: Bill was a master swordsman. He was trained by Hatori Hanzo and his name alone was enough to convince Hanzo to make Beatrix a sword, implying his sheer skill. In his showdown with her, he disarmed Beatrix within seconds.
- Martial Arts: Bill was most likely an expert martial artist as he was trained by Pai Mei (and evidently held his own against the aged master upon delivering Beatrix for training).
- Miscellaneous skills: Aside from these skills, Bill was also shown to be highly intelligent, planning his crimes and Beatrix's movements in advance, as well as creating a powerful truth serum. A true renaissance man, he was able to seamlessly transition from storytelling or flute-playing into sudden violence. Despite being no longer in his prime, he was one of the most formidable characters in the movies.