He was developed by Joss Whedon and portrayed throughout the television series by Nicholas Brendon and in two episodes by his twin brother, Kelly Donovan.

Xander is one of several friends of Buffy who assist her in saving the world against numerous supernatural events that plague Sunnydale, California, a town built over a doorway to hell.

However, Xander's unique position as both outside and within the main group enhances his insight into the other characters, especially as the series continues; in Season Eight, he is Buffy's unofficial "Watcher", who watches over her and has a clear perspective.

The character of Xander Harris also appears in numerous items in the series Expanded Universe, such as comic books, tie-in novels and video games.

Xander meets Buffy Summers (Gellar), the Slayer, on her first day at Sunnydale High, as well as her Watcher Rupert Giles (Anthony Head).

In the season finale, "Prophecy Girl", Xander saves Buffy's life by administering CPR after she is drowned by the Master (Mark Metcalf).

In season 2 (1997–1998), Xander begins a turbulent and ambiguous relationship with popular girl Cordelia Chase (Carpenter) after they are thrown together in several life-or-death situations.

In the season finale "Becoming, Part Two", Xander decides not to tell Buffy about Willow's plan to re-ensoul Angel—who has lost his soul, reverted to his vampire persona Angelus, and is going to bring about the apocalypse— so that she will not hesitate to kill him in order to save the world.

A solo adventure begins when the rest of the Scoobies insist he stay away from the dangerous fight with the Sisterhood of Jhe; he borrows his uncle's classic car, loses his virginity to mentally unstable rogue Slayer Faith (Eliza Dushku), and single-handedly stops a band of zombies from destroying Sunnydale High School.

In the season 4 climax "Primeval", Xander becomes the "Heart" in the spell, which conjoins him with Buffy, Willow and Giles to defeat Adam (George Hertzberg), a part-demon, part-human cyborg monster.

Xander's doubts about his future with Anya are expressed when he summons the all-singing demon Sweet (Hinton Battle), in the musical episode "Once More, with Feeling".

In the episode "Dirty Girls", Xander's left eye is gouged out by the evil preacher Caleb (Nathan Fillion), and he begins wearing an eyepatch.

Between 2001 and 2004, Joss Whedon and Jeph Loeb developed a 4-minute pilot episode for Buffy the Animated Series, which was set during the show's first season.

In its final arc, "Last Gleaming" (2011), Xander witnesses a possessed Angel kill Giles in the ruins of Sunnydale, as well as Buffy's destruction of the source of all magic.

Subsequently, he and Dawn settle down and move to San Francisco, where they allow Buffy to stay with them while she waitresses and hunts vampires by night.

In "The Watcher" (2013), the season's villains, mystical 'Siphon' Severin and rogue Slayer Simone persuade Xander to assist them in their plot to turn back time and avert Twilight.

Acting on the same information, Buffy and Willow decide that the Deeper Well in England—a prison for ancient demons—may contain enough magic to save Dawn, and Xander travels with them.

While entering the Well, Xander confesses his betrayal to Buffy and informs her of Severin's plan to use the magic in the Well to turn back time, which could destroy the universe.

He stands beside Buffy when they discover Simone's ulterior motive for entering the Well; she has allowed herself to be sired by the very demon which created the first vampire, transforming herself into a powerful vampire/Slayer hybrid.

In Buffy the Vampire Slayer Season Ten (2014–16), Xander has more of a starring role, and the first arc, "New Rules," is co-written by his former portrayer, Nicholas Brendon.

He does not know, as Dawn later confesses to Buffy, that she is experiencing intense emotional anguish from reliving her entire life at the moment Willow brought her back.

Chaos Bleeds prompted prequel comic books published by Dark Horse and a novelisation, which treats it as a "lost" episode of season 5.

For both Xander and Ron, their comparative lack of special gifts "accentuates the loyalty and bravery... [each] offers as a friend" given that the characters often place themselves in mortal danger.

The episode depicts Xander single-handedly saving the school, and potentially the world (given the Hellmouth beneath it) from an explosion in simultaneity against glimpses of the other Scoobies' struggles with the hydra demon.

Wilcox argues that the basement is a representation of the subconscious, and that young carefree zombie Jack O'Toole (Channon Roe) represents Xander's id, his inner desires.

As the "warrior of words", Wilcox feels that this makes Xander the character who most clearly represents series creator Joss Whedon.

In "Homecoming", he admits to swallowing his pride to borrow a tuxedo from his cousin Rigby, from a wealthier branch of the family, who, as Xander puts it "shun us... as they should".

His dissatisfaction reaches a nadir in season 4: Xander's physical dislocation from his friends, who are now in college, leads to a profound sense of being useless.

(This position of being less "chosen" for social inclusion and great things than his peers makes him uniquely able to comfort Dawn in season 7's "Potential".

Jowett attributes this to Xander's internal struggles (which concern a crisis of masculinity and his role amongst his friends) do not contain a readily accessible solution for viewers to hold onto.