Buffy the Vampire Slayer was notable for many things: subverting horror tropes, representing trials through monsters, and well-written characters, among other things. No one can deny the influence that Joss Whedon's first big television show had on pop culture in the '90s.
Spike (James Marsters) quickly became a fan favorite after his first appearance in the Season 2 episode "School Hard", as much as it derailed Joss Whedon's vision for the character as a one-off bad guy. Even people who have never seen the show may recognize him, so saturated was the media with the relationship between him, Buffy Summers (Sarah Michelle Gellar), and fellow vampire Angel (David Boreanaz).
Redemption Arc
Over the course of Buffy's seven seasons (and a spin-off) Spike gradually moved from a murderous, soulless villain (literally) to a reluctant ally, to someone who fought to become a better person. After an arc of increasingly well-meaning deeds (and more than a few horrible ones) he realized that the only way to become someone Buffy deserved was to retrieve the soul and humanity he'd lost when he was turned.
His final act of the series was to sacrifice his life destroying Sunnydale Hellmouth, and though he later popped back up in the spin-off Angel, it was still a great way to go out.
A Memorable Entrance
While Season 1 of Buffy the Vampire Slayer was engaging and a good introduction to the series, Season 2 upped the stakes early on with the introduction of Spike. With the current villain of the Ascended One, an eternal child whose clear non-eternalness was becoming clear to the producers, creating problems for the story, they decided to replace him in the most sudden way possible. Spike picks him up, dumps him in a cage, burns him in the sunlight, and declares himself the new villain in a matter of minutes.
Also, crashing a blacked-out car into the town's welcome sign was an instantly memorable moment.
His Relationship With Angelus and Drusilla
A being who has been around for nearly 200 years understandably has a lot of history with a lot of people. Spike's relationship with his sires Angelus (David Boreanaz), one of the most infamous vampires in Buffy the Vampire Slayer's history, and Drusilla (Juliet Landau) is tense and long, and not nearly enough of it is seen on-screen. Spike reappears in Angel's Season 5, where audiences get even more insight into his relationship with the other two.
A love triangle was bound to show up sometime in this WB drama, and the fact that all three of the characters involved are vampires from the 18th and 19th centuries adds an interesting twist.
He Always Has A Witty Quip
Spike never let anyone have the last word. Whether it was quoting Nirvana before a fight, throwing out sarcastic banter with Buffy, or insulting everyone in the room at the drop of a hat, he had a tendency toward blunt honesty that was as entertaining as it was needed.
He was the cause of many of the later seasons' funniest moments, taking over from the show's previous funny bone, Xander (Nicholas Brendon) (although, as seen sadly too little, the two of them made hilarious comedic pairing).
His Punk Aesthetic
A vampire who, instead of silk and mansions, prefers leather duster jackets, Sex Pistols, and hanging around crypts? Spike was a breath of fresh air among vampire media insisting on depicting them the way Anne Rice did--which was understandable considering Rice's immense popularity at the time.
Audiences saw a great contrast between him and Angelus in flashbacks–Angelus presenting himself as more of a stereotypical, Lestat-like figure, with Spike serving as a more malicious Louis. They were each as sadistic as the other (Angelus perhaps more so) but Spike seemed altogether less concerned with riches and finery.
He's Evil, Except When He's Not
Spike had plenty of soulless moments, but he also showed a surprising amount of humanity and sheer emotion that other soulless vampires rarely did. Spike was also oddly perceptive throughout the show, often offering some insightful comment about another character's problems, though rarely in the most tactful way.
Much of the time, these comments actually served to push the character to realize or confront their issues, making Spike a useful character for the writers to play around with.
Chemistry With Buffy
Season 6 sees Spike and Buffy take a further step in their antagonistic relationship as they begin a contentious physical relationship. Although discussion has reigned among the fandom for decades about the dubious health of their relationship, particularly in the depressing Season 6, no one can deny that every between them crackles with chemistry.
Whether they were threatening to kill each other, sharing a tender moment of compassion and empathy, or protecting each other from danger, Spike and Buffy were one of the most iconic and intriguing parts of the later seasons.
Love Drives All His Actions
Spike may not have had a soul as a vampire, but that didn't mean everything he did was sadistic. His initial turn from human to vampire was inspired by his sorrow at being rejected by the woman he'd expressed feelings for, and afterward, he formed a strong and loyal bond with Drusilla that lasted centuries.
While many of his actions without a soul were driven ultimately by selfishness, his choice to find and fight for a soul was monumental in his character arc. In Season 7 audiences are introduced to a Spike who is able to act out of true selflessness and care for the ones he loves.
Fleshed-Out Backstory
In "Fool For Love" fans get their first look at who Spike was before he was turned into a vampire, and it wasn't what anyone would expect. "William the Bloody" was in fact a lovelorn nobleman known for his horrible love poems. Later, Season 7 provided even more sympathy for him by revealing that one of his first acts as a vampire was to turn his mother and cure her of illness. This backfired, but it's the thought that counts.
Spike became one of the most interesting and nuanced vampires in the series, alongside his rival Angelus, and his long history became a driving motivation for the actions he took on-screen.
A Charismatic Performance
The character of Spike couldn't have been performed by anyone other than James Marsters. He brought excellent comedic timing, 35 years of acting and producing experience, and a passion for the character to the table.
Spike's actor showed a real understanding of the character even early on in the story, infusing him with charisma and entertainment, and it's hard to imagine anyone being better for the role. He’s also been outspoken about supporting his fellow coworkers in the aftermath of the accusations of a bad workplace environment behind the scenes of the show.