Xander Harris is the sidekick every slayer wants and the best friend every high school teenager needs. In Buffy the Vampire Slayer, Xander was a quick fan favorite with his witty one-liners and sarcasm. Xander always provided comedic relief, but was also surprisingly caring and emotional. Xander loves his friends, especially Buffy and Willow and will do anything for them, and over the course of the seven seasons this television show was on air, Xander does do just about anything for his best friends, he even manages to save the world a few times.
Since the series' finale in 2007, Xander has become the subject of many social and Hunan psychology studies. He's an intriguing character and one that the show's creator, Joss Whedon, based loosely on himself and his own high school experiences.
He Gains Confidence
Xander starts out as a young, self-deprecating teenage boy who uses sarcasm and humor to deflect his real emotions. He carries that same wit and humor throughout the series, but he stops using it as a shield and starts embracing his emotions.
Xander gets pretty caught up in the idea that he's not masculine enough and frequently finds himself intimidated by Buffy's strength and Willow's intelligence, but when he realizes he has other things to offer the Scooby gang and is just as valuable as everyone else, he gains his confidence and because even more of an asset when it comes to saving the world.
He Becomes A Solider
In seasons two's Halloween episode, Buffy and her friends decide to dress up for Halloween and volunteer taking kids around Sunnydale for trick-or-treating. The Scoobies get their costumes at a new shop called "Ethan's". Xander picks out an army costume in an attempt to look tough and masculine.
What the group doesn't know is that the costumes have been cursed and they will become what they are wearing on Halloween. Xander becomes a solider, equipped with weapons and the knowledge to use them. After the curse is broken and things go back to normal, Xander has retained all of the military knowledge, which comes in handy a few times throughout the rest of the series.
He Matures
When it comes to dating and women in general, Xander struggles. In the first season, after Xander meets Buffy, he develops and instant crush on the slayer, but those feelings are not reciprocated, so he's left pining for her secretly. When Xander finally moves on from that crush, he falls for Cordelia, but they have to keep their relationship a secret because of the social hierarchy at Sunnydale High and the fact that Cordelia's friends would disown her if they ever found out.
Then, Xander has a secret affair with Willow while dating Cordelia. Xander seems to choose women he can't have for the first couple seasons of the series, but that does change and Xander starts to realize his worth as a partner and what he can bring to a relationship.
He Finds True Love
For the first time in Xander's life, a girl finally has a crush on him. That girl is Anya, the vengeance demon, turned high school student. Anya shares her feelings for Xander, which initially he doesn't reciprocate, but the more time he spends with her, the more he starts to fall. Xander and Anya get engaged at the end of season five.
Xander proposes with a beautiful speech leading up to another potential apocalypse. The two stay plan their wedding in season six, but Xander can't go through with the wedding, letting some of his old fears back in, he leaves Anya at the alter. The two do get a happy ending, though, and somehow manage to find their way back to one another because Xander knows he'll never love anyone the way he loves Anya.
He Gets Braver
For a guy who didn't even know vampires were real before Buffy came to town, Xander adapts rather quickly to the idea and has the series progresses, we see him getting braver with every big-bad encounter he has. In season four, the Scooby gang must face Adam, who is a demon-cyborg created by mad scientist and college professor, Dr. Maggie Walsh.
Because Adam was built from different demons in a lab, his strength is unlike anything Buffy has ever faced. In order to defeat Adam, Willow casts an ening spell, which gives Buffy different aspects of her friends, which make her stronger. Xander's contribution to the spell is his bravery because he has proven so many times that he really isn't afraid of anything.
Finds His Independence
In the beginning, Xander can be classified as a "sidekick". He's the guy who is always one step behind everybody else and ready to jump in if he's called to do so. In later seasons, especially once the gang has graduated from high school, we see Xander off on his own a lot more. Willow and Buffy choose college after high school, but Xander gets a construction job instead.
Xander also goes off on his own adventures a little more in these seasons, even fighting demons he comes across when Buffy isn't around. The best part of Xander's new found independence is that he embraces it. He stops feeling left out and starts feeling self-sufficient.
Gains Trust
There are times when Xander does more harm than good. Early on, he has a tendency to get in the way or cause more trouble when Buffy is slaying, so much so, that in one episode called, "The Zeppo", Buffy asks Xander is hang back when she is patrolling because she's sick of having to save him all the time and when she has to do that, it jeopardizes the current mission. Over the course of the seven seasons, Xander proves himself invaluable to the team.
Toward the end of the series, Buffy goes to Xander with a special task. She asks him to get Dawn out of Sunnydale to protect her from looming danger. Buffy asks Xander to do it because he's the only one she trusts to look after her sister.
He Saves The World
Xander is always willing to lend a helping hand when Buffy needs it while saving the world, but he his solely responsible for saving everyone at the end of season six. Willow has become Dark Willow after Tara was killed by Warren.
Dark Willow goes on a vengeance spree and kills Warren, but when that doesn't heal her pain, she decides that she must end the world to make what she's feeling go away. As Dark Willow summons all of her power, Xander finds her and tells her how much he loves her. Xander refuses to leave Willow's side and his love for her is what stops her and saves the world.
He Becomes A Heartthrob
Xander's character is clearly set up in the beginning to be a typical high school nerd. He seems to embrace his social status with jokes at his own expense and keeping within the boundaries his status has created.
Toward the end of high school and into his adulthood, Xander erases those boundaries and as he becomes more confident and unwilling to be cast aside, he also becomes more of a lady's man. Cordelia seems to be the first girl who notices Xander's growth and the two start dating. There are also a few episodes where Xander becomes the subject of multiple women's affections.
He Makes Sacrifices
You could argue that Xander doesn't start out as the most selfless character. In the early episodes, he's frequently seen putting himself and his safety first. Though he loves his friends, he's selfish and he tends to use Buffy and Willow when he needs them and doesn't seem capable of returning the favor.
That all changes as Xander grows up and we see him make a lot of sacrifices for the people he loves. In the seventh season, during a fight with the wicked Caleb, Xander loses an eye. It's a painful sacrifice, but one Xander makes willingly to help his friends and to keep others safe from harm.