Look, I'm never gonna be able to explain this story, let alone write all of it down, so here's the highlights- it starts, of course, with a spell gone wrong (doesn't it always?) and accidental baby acquisition. That's October, 1980, and Rupert Giles' life gets turned upside down because his crazy, doesn't-even-like-children idiot of a boyfriend decides to keep it.
(Maybe it starts a little bit earlier, when neither of them leave after Randall dies, but really, it starts with baby Cedric, clearly a newborn, shrieking whenever Ethan tries to put him down. Not even cold, guarded, selfish Ethan can walk away from that. And Ethan in 1980 is so much less guarded, so much less selfish than the Ethan that showed up in Sunnydale to open a Halloween shop.)
Within a week, there is no going back. So Rupert starts trying to figure out what Ethan did, and how, and it takes a while, but by the time Cedric is six months old, they've worked out what happened. Which is nearly a miracle, given that Cedric still will not sleep unless he's being held, though he will tolerate being put down if he's not tired. The spell, apparently, had created a genuine genetic child from them. It was nothing like what Ethan had meant to do, and yet he couldn't find it in him to regret it.
Within a year after figuring out the spell, they've decided- irrationally, and mostly fuelled by sleep deprivation and the way Cedric's presence makes Rupert's father stop making snarky comments about Ethan being a bad influence- to consider doing the spell on purpose.
And so came Percy, just barely two years younger than his brother and so wildly different. Where Cedric was clingy and demanding, Percy was calm. Where Cedric was adventurous- especially as they grew- Percy was more calculating, likely to assess before diving in.
And a few years after Percy came Izzy, quiet, gentle, and incredibly cuddly, and far more fond of trailing after her brothers than either of them really appreciated.
And all of that? That's not even the real story. The real story starts when they get to Sunnydale, and Cedric- smart, loyal, stubborn Cedric- meets Buffy Summers and changes his whole life plan in order to keep her safe.
Ok but also... the original concept for Cedric came about because I kept stumbling over people comparing Spike to either Giles or Ethan, and usually the latter was presented as an explanation for why Spike drives Giles up a wall so easily. And I had a thought, while contemplating this, that really it's not that Spike is entirely like either of them, it's more like he's... sort of not unlike what you'd get if they had a kid, but, like, he's what Giles would think is the worst outcome of that... so what would be the best outcome?
Well. It was supposed to be Cedric. Cedric is frequently one bad paragraph away from being a gary stu... but Cedric also turned out to be an emotionally barely-stable, clingy, desperate disaster. So really, the best outcome is Izzy, but Izzy wasn't even part of the original story concept- in that one, Cedric was an only child- but, of course, I couldn't resist contrasting Cedric with more kids, and besides, so much of his personality was just... obviously an older brother, like. I can't explain it but everyone knows what I mean.
Anyway, the original concept was, ok, I was sort of halfassedly outlining a story where Giles and Ethan get back together post-Chosen, and just as they start to settle into being together again, there's an Incident with weird magic portals and alternate universes and shit, and suddenly there's this kid- about a year older than Buffy- stranded there because his universe is just... gone. So Cedric started out very broken, lost, and needing his dads to be his dads when they had no idea how. And he was in love with Buffy, because of course he was.
That's not the final concept for the Cedric 'verse, though, but it is one I'll probably play around with again, because stranded!Cedric is so much fun to write, and so heartbreaking.
The current (final) concept is just... an AU of the Buffyverse, with Cedric. I'm still tooling around with what his presence changes, and what it doesn't, and how much I'm willing to break the poor kid, and whether I'm even writing anything in this verse or just gonna keep not!ficcing it... But there we go.