Rain is a water ghoul that used to live in more temperate waters, so during the coldest days of the year he would usually retreat to the cave of hot springs near the Ministry by swimming through an underwater passageway.
But one time, he found that the entrance had been blocked by a partial collapse, and he absolutely refused to try and clear the rocks in these frigid waters, so he returned dejected and ice-cold shivering back to the Den. Then, in a last ditch effort, he decided to find Dewdrop.
At first, he’s a bit hesitant because even though Dew’s the resident fire ghoul, everyone has learned to not really bother him when he’s practicing (and plus the other ghouls can withstand the cold better than Rain anyway). But when Rain turns the corner into the practice room, and awkwardly sidles up behind Dew for warmth, Dew is really understanding and fr offers to take a bath with him. (Rain would’ve done that but the water goes cold quickly yk)
So then, they’re just both in Rain’s bathtub; and Dew’s warming the water up until the room is steaming, and Rain is pressed so closely against Dew that if he didn’t have gills, he basically wouldn’t be able to breathe between Dew’s shoulder and having half his head underwater, and the only reason his purring dies down is because Rain falls asleep, blissfully no longer shaking uncontrollably, and Dew just accepts it with a smile. He deliberately makes the parts of himself not directly touching Rain hot enough to burn to maintain the water temperature, and keeps other areas of himself a suitable ‘living heater’ temperature. It’s not like he minds being in the water for a long time anyway.
Like three hours later, one of the other ghouls comes to find them for dinner and finds them both asleep in the tub completely wrapped around each other.
And so was born an annual tradition. Even when summer came, Rain never ended up clearing the blocked entrance because he didn’t need (want) to.