Xander was immediately taken with Buffy when she first came to Sunnydale. Throughout season 1, he subtly tried to show her how he felt about her..for example, in The Witch, he gave her an ID bracelet. Unfortunately, Buffy didn't return his affections, as she was too intrigued by Angel to think of Xander as anything more than a friend. She tells Xander this in Prophecy Girl.
It's in Prophecy Girl, though, that Xander is single-handedly responsible for saving Buffy's life. He's the one who forced Angel to show him where The Master's lair was, and he's the one who revived Buffy after she drowned.
In Season 2's When She Was Bad, Buffy's still traumatized by the events of killing The Master, and she acts out by pushing Angel away and making a play for Xander. Actually, she pretty much teases him rather than makes a play for him. She does a sexy dance around him on the dance floor of The Bronze, then whispers to him, "Xander, did I ever thank you for saving my life?" When Xander says no, she replies, "Don't you wish I would?" Then she walks off.
It's not until Inca Mummy Girl that Buffy acknowledges that Xander brought her back to life.
The only time that Buffy appears to be interested in Xander is when she's under the obsession spell that Amy Madison cast in Bewitched, Bothered & Bewildered. She comes on to Xander big-time, and he narrowly escapes her advances, telling her that if she had one clue what it would mean to him....but since she doesn't, he can't be with her. She remembers this when the spell is reversed, and one wonders if anything could actually ever happen between them.
Even though Xander is now seeing Cordelia, it's apparent that he still has a yen for Buffy....plus, he's still saving her life every so often, as he does in Go Fish.
But you gotta wonder what's going to happen if Buffy ever finds out that Xander didn't tell her that Willow was going to try to restore Angel's soul again...
Season 3: Well, it appears as though there's a slight rift in Buffy's relationship with everyone..time'll iron it out, I guess. Anyhoo, in Dead Man's Party, Xander really lays into Buffy...he calls Buffy an idiot and that sort of thing, basically tells her she was stupid to leave. We don't actually see the relationship repaired a whole bunch, but they do compliment each other's fighting tactics when the Zombies come a-knockin'. So...the groundwork's set, and maybe by next episode they'll be a little more relaxed with each other.
Update!: Been a while since I wrote on this arc, but...there've been some ups and downs. Xander and Buffy were okay with each other until Revelations. That's 'cause this is the episode where Xander learns that Angel's back..and Buffy knows it! He says some nasty things to Buffy about everything, and even goes so far as to tell Faith where Angel is so she can kill him. But when it gets down to the wire, Xander does tell Buffy about it so she can stop Faith.
The next big thing for Xander and Buffy happens in Amends. Maybe it's the holiday spirit, or maybe he just wanted out of the house, but it's here that Xander offers his help in researching what's up with Angel. He admits to Buffy that he hasn't been their biggest advocate in the past, but he does want to help. That's all for now.
