Sweet Rein Vol. 1 (manga)

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?!