Santa Claus Dreaming.

Koshiba Seijuro is the assisstant to some influential figure whose daughter was all messed up, and who comes to the psychiatrist Koizumi Santa and asks him to counsel the daughter without her knowledge. Since Santa is a compulsive gambler he's hard up for money and takes the job even though he's queasy about the ethics. Turns out that she's the same girl he took to the hospital nine months earlier when she had passed out drunk on a roof preparing to jump. As it happens she woke up in the hospital and saw Santa's best friend Kusanagi Shunsuke (a surgeon) holding the Santa Claus hat Santa had been wearing (it was Christmas Eve), and it turns into a very complicated love triangle.