Bill

Physical appearance
Bill is played by David Carradine (in his 60s during filming), and his look is iconic and deliberately "retro-vintage":

Hair: Long, straight, silver-gray (almost white in some highlights), styled back or loose over the shoulders. It gives him an air of a wise old man/decadent hippie rocker.

Face: Wrinkled, with sharp features, piercing eyes (sometimes sad or melancholic), thick eyebrows, and an expression that shifts from serene to threatening in seconds. He has a warm smile that contrasts with his dangerous aura.

Typical attire: Elegant yet relaxed clothing—open silk or linen shirts, velvet or leather jackets, loose trousers. In the final scene, he wears a casual beach outfit with retro touches (yellow shirt, etc.). He always looks like a sophisticated criminal gentleman, not just any thug.

Height and build: Tall and thin (Carradine was around 1.85 m tall), with an imposing presence despite his age. He walks calmly, almost like a dethroned king.

Bill's Personality
Bill is one of Tarantino's most complex and charismatic villains. He's not a cold, emotionless psychopath like some of his other antagonists; he's much more sophisticated and multifaceted:

Intelligent and cultured: He speaks fluently about Chinese mythology, philosophy, and superheroes (his famous Superman and Clark Kent analogy is legendary), plays the flute, tells epic stories, and has a rich vocabulary. He is a worldly, educated, and thoughtful man.

Charismatic and paternal: He is affable, polite, calm, and even tender at times. He treats his subordinates (including Beatrix) with respect and warmth, like a father or mentor. It's easy to understand why Beatrix fell in love with him.
Cruel and possessive: Beneath that facade lies an extremely masochistic and vengeful side. He cannot tolerate rejection (the wedding massacre is an "overreaction" that he himself coldly admits).

He is manipulative, jealous, and capable of brutal acts without deep remorse. His moral code is twisted: he kills without hesitation, but he does so with "elegance" and his own rules.
Pragmatic and lethal: As the squad leader, he is calculating. He prefers simple and effective solutions (shooting Beatrix in the head rather than fighting honorably). He combines elegance with subtle menace.

After the massacre
Beatrix fell into a coma for four years. Bill took their daughter (whom he named BB), raised her in secret, and told her stories about her mother as a kind of tragicomic tale. Meanwhile, he continued living his life as a sophisticated criminal and murderer.
The end

In Volume 2, Beatrix (now awake and having killed almost the entire squad) reaches Bill. They face off in an epic duel (sword vs. sword), but also have long, philosophical, and emotional conversations. Bill is a complex villain: cultured, charismatic, and somewhat paternal, but also extremely cruel and possessive.

Finally, Beatrix kills him using the secret technique Pai Mei taught her: the five pressure points to make the heart explode. Bill takes his leave gracefully and dies after walking a few steps before collapsing.

Origins and formation:
Bill grew up without knowing his biological father and sought father figures throughout his youth. He was protected and mentored by individuals like the pimp Esteban Vihaio. He also trained with the legendary Chinese martial arts master Pai Mei, who subjected him to brutal training (such as climbing thousands of steps carrying heavy buckets of water). This transformed him into an extremely skilled, disciplined, and lethal warrior. He mastered kung fu, samurai swordsmanship, and possessed a broad knowledge of culture (he could recount ancient Chinese myths, play the flute, and so on).
Leader of the Assassin Squad
Bill became the leader of the Deadly Viper Assassination Squad (DiVAS), an elite group of contract killers. He was known by the codename Snake Charmer.

The main members were women (except for her younger brother Budd):
Black Mamba (Beatrix Kiddo / The Bride)
California Mountain Snake (Elle Driver)
Copperhead (Vernita Green)
Cottonmouth (O-Ren Ishii)

Relationship with Beatrix Kiddo (The Bride):
Beatrix was his most talented student, his right-hand woman, and his lover. They had an intense and passionate relationship. When Beatrix became pregnant with his child, she decided to abandon her life as an assassin and start over: she ran away, changed her identity, and became engaged to an ordinary man in Texas to marry and raise her daughter in peace.

The wedding massacre:
Bill refused to accept that she had left him (much less that she had married someone else). In his most masochistic moment, he led the attack during the wedding rehearsal. He and the squad killed almost everyone present (including the groom and the guests). Beatrix was brutally beaten, shot in the head, and left for dead. Bill personally fired the final shot into her head, though before she had revealed to him that the baby was his.