Prologue: A Creation Story of Sorts
At the beginning of existence, there was just chaos. It wasn't unpleasant, but it was…well, chaotic. Physics and time were there if you wished to dabble in them, but it was hardly the requirement it is today. And while it certainly gave everyone the freedom to do whatever they wanted, it also made it extremely difficult to get anything done. So some of the more prominent forces floating about decided it would be a good idea to put some sort of rules in place, even if they left loopholes for them to be broken.
So two of them made the concepts of a physical reality and the intangible movement through a fourth dimension mandatory for most of everything, most of the time. It worked pretty well, and after a while, some of the more sentient of the creatures gave them names like Mother Earth and Father Time. Which was alright by them. They were powerful and established enough that names didn't really affect them anyway.
But as with any system of rules, other side effects and problems emerge. Like, because time was a definite thing now, there was a limited amount of it allotted to each thing designated as "alive". And that allotted time had a beginning and an end. And because of that, Life and Death also became a definite thing.
The funny thing about us sentient creatures is that we tend to be a bit give or take in how much we embrace the rules. Ever since consistency was developed, humans especially have grasped it heartily, and tend to find deviation from it very uncomfortable. The way Death tends to be portrayed as a menacing figure is a great example of that. But some of the smaller changes we expect in our allotted time have caught our collective interests as well:
The passage of day and night and the time we use to rest, recoup, and regain energy for the next day turned into the Sandman.
Watching the life and death of the differently-sentient plants and animals around us in the changing of the seasons transformed Life into the Easter Bunny after a handful of symbolism was mixed in.
Death, on the other hand, went unchanged, and Jack Frost took up the task of bringing the dead time of the year when the earth was allowed to rest.
One of the first heralds that you are physically growing up (and before the less marketable teenage years) eventually, after much deliberation and changes, became the Tooth Fairy.
And the ever-fluctuating nature of our bonds to those we care about and/or loathe became Cupid.
As more and more of these bureaucratic or guardian figures emerged, the more they realized that to function in this less chaotic system they've created, they needed to organize themselves. At least a little bit. And out of this, the Council of Legendary Figures was born, each in charge of their own little aspect of existence.
The last of the Legendary Figures to come into existence is the resilient spirit of determination humans have, even when faced with what they know, for a fact, is going to be a bad time. In many of the colder areas of the world, humans would have some sort of celebration before the worst of the winter set in. "Welp," many of them had said to themselves, "we've finished the work we needed to get done in the fall. Now we've just got to settle in for the long haul until spring shows up again. Any suggestions how to pass the time?"
And someone, as to be expected, suggested, "How about a party?"
This suggestion was usually accepted. And it's gone through many many iterations and traditions have changed many many hands until it reached the commercialized December holiday we know and love today.
Most of the Council accepted this new figure with open arms. One tried to but found his existence grating and just a tad insulting to his own. "Great," thought Jack Frost, "Humans needed to create this caricature just to bear the concept of me."
Legendary Therapists were not quite a thing yet, so this resentment went ignored and allowed to fester until even he forgot why he disliked the jolly fat man. He just knows he does, and he must be justified in that, right?