Xander

Name: Alexander ‘Xander’ LaVelle Harris
Type: Human
History: Xander was born and raised in Sunnydale, and has been friends with Willow since childhood (they used to sleep over at each other's houses regularly). Jesse was Xander's other best friend, until he was turned into a vampire and inadvertently staked by Xander.
As with most of the characters, little is known about Xander's family, except that his mom can't cook, there could be alcoholism in the family (he sleeps outside on Christmas Eve to avoid his family's drunken fights), and Cordelia has implied that his father is unemployed.
Despite his problems (or perhaps because of them), Xander consistently maintains his sense of humour, making wise-crack after wise, no matter what the situation may be. Xander was not an exceptional student, but he's always been ready and willing to help Buffy in her endeavours.
His track record in the romance department has been less than optimal — he was turned down by Buffy, fell for a praying mantis and an Incan mummy girl, had a one night stand with bad-girl Faith (only to be kicked out afterwards and assaulted by her shortly after), and went to the prom with an ex-demon (Anya). When he settled into something vaguely resembling a relationship with Cordelia, he messed that up by kissing Willow ... and getting caught.
Xander did manage to graduate high school (despite some fan speculation that he might not), although didn't go to college with the gang, instead successfully working in construction, where he is now making pretty good money.
He and Anya became a couple after high school, and eventually got engaged in Season 5. Unfortunately, Xander wasn't ready to get married, due in large part to a fear of becoming like his dad, and he called it off on the day of the wedding, breaking Anya's heart.
After Tara's death, Willow went berserk, and now endowed with immense magical powers, attempted to destroy the world. Although Buffy was no match for her, Xander managed to get through to Willow by reminding her of their close friendship over the years. Xander thus saved the world.
In season seven, the gang realised that Xander is the glue that holds them all together, and when he lost one of his eyes to the evil priest Caleb, the glue became unstuck, so to speak.