Botched summoning spell (referencing some kind of soldier) gets Zack in the Buffyverse. Buffy is immediately suspicious, naturally. No one can be that nice and cool. Also the strength implies some kind of demon to her and he pings her slayersenses as faintly unnatural somehow.

But zack being zack worms his way into the group and slowly wears her down. it helps that he's admirative of her abilities, but not in a OOOOH A *GIRL* CAN DO THAT insultingly disbelieving way either. he doesn't try to coddle her, which is pretty rare in the relationships she usually has with guys. he just backs her up.

then sephiroth feels that zack is alive again *somewhere* and punches through dimensions to find him, alas this is crazy!jenova!sephiroth we're talking about and it leads up to a very heartbreaking battle where zack has to kill his ex-best friend turned insane destruction god.

if i'm feeling generous I might get the scooby gang to save him somehow, and then they'd have to deal with learning to live in the buffy world and how weird that is and repairing their friendship and falling into bed. XD Buffy would be wary and very "you hurt him, i break you" at seph, which baffles him a bit but that's not a bad thing, he needs to be off balance more often XD