Yoroshiku Master Series by Sakura Tsukuba, 筑波 さくら

Sad at the thought of spending Christmas alone, Kurumi Sagara goes out for a walk. While she’s crossing the street, a boy bumps into her, and a rein suddenly appears that binds them together. The overjoyed boy tells her she’s his master and that she’s a Santa Claus. Kurumi dismisses him as a crazy person, but then he transforms into a reindeer?!

Kurumi and Kaito have remained together for a year, but a Dark Santa appears, intent on breaking the two apart. Kurumi and Kaito's rein is severed, and the Dark Santa takes control of Kaito. Can Kurumi manage to get Kaito back, even when she’s still unsure if he's truly happy as her servant?

One day when 17-year-old Sagara Kurumi accidently bumps into a guy on the streets, reins suddenly appear and binds them both! What is this?! Things get even more confusing when the boy, Kaito, clasps her hands and calls her his master! When Kaito transforms into a reindeer it just becomes too much for poor Kurumi who's out as a light. But it does indeed seem like Kurumi's a santa and Kaito's her reindeer and that they've got an important job to do on Christmas Eve