LogFAQs > #967760730

xander appears in every episode of buffy except 1
so only buffy and willow appear in every single episode of the show

xander was planned to be in the episode he missed out on though and it actually went back to the jessie character.
see there were a couple of plans with jessie that never happened

originally joss whedon wanted jessie in the opening credits for the first 2 episodes so it would be a shock to fans when he died. but this idea was nixed cause they would need to pay editors for 2 different opening credit scenes and they werent willing to do that.
this idea was later used in season 6 when they added amber benson to the opening credits for the episode her character died in. though her character tara was basically a series regular in spirit just not in pay cheque. she appeared in nearly every episode of seasons 4 5 and 6 and even got an episode centered around her.

anyway the episode that xander missed was the one where the first evil appears to characters as deceased characters for the first time and tries to manipulate them.
well xander was supposed to be in the episode and the first would have appeared to him as jessie
i dont remember why this plan was scrapped.
but i do know they wanted amber benson to come back for this episode and have the first take on her form to hurt and manipulate willow and amber benson refused because she felt it was to cruel to willow and disrespectful to her characters fans.

there actually were plans to revive tara in the final season but they fell through due to scheduling conflicts