Naruto himself is likely to be the focus. His difficult childhood and his longing for family make him a prime candidate. Sasuke could also be a focus, as his quest for revenge often led him to be alone and Christmas could be a time when his isolation is highlighted. Hinata might be another, as her unrequited love for Naruto could lead to some sad Christmas scenarios." "Answer2": "Definitely Naruto. He has always been an outcast in the village for a long time, and Christmas could be a time when he feels that even more. Sakura could be in the spotlight too. Maybe she misses Sasuke during Christmas and it makes her sad. And Kakashi, with his past traumas and the fact that he has lost so many people, his loneliness during Christmas can be a great focus in such fanfictions.
Well, Naruto is an obvious choice. His story of growing up without parents and always striving for acceptance makes for a sad Christmas story. Gaara is also likely to be a focus. He had a very troubled past and was feared by others, so Christmas could be a time when he reflects on his loneliness. And Jiraiya, as he was often away from Naruto and had his own regrets, could be a central character in a Naruto sad Christmas fanfiction.