Sakura Tsukuba's Sweet Rein tells the story of Kurumi, a teenage girl who discovers that she's a Santa Claus when she encounters Kaito, a wispy and beautiful boy who is also sometimes a reindeer, and who is quite literally bound to her with an invisible rein that compels him to obey her commands. Also, they are in love.
It is, without question, the single most bonkers premise I have ever encountered in a lifetime of reading Christmas comics, and I've saved the second volume for an entire year waiting to read it. And folks... it does not disappoint.
Because this is the one with Dark Santa.