I added a story called the Snow Demon's Daughter which is about a Yuki-Onna who is half demon and half human. It would be fun to create an artist trading card for. I wrote this story a good five years ago for a class, so I don't think it's my best work. I just wanted to update more often, and I am busy with my Excelsior script.
I'm hiding the Author's Comments about the Snow Demon's Daughter, so when you click "Read More" make sure you've already read the Snow Demon's Daughter or be ready for spoilers.
Update: Added picture for the Snow Demon's Daughter. (May 27, 2015)
The first is when I started out writing it, I really wanted people to be surprised that Yuki was the demon half of the personality. You see, Yuki is a Yokai. She was prepared for the power of the Yuki-Onna and, more importantly, the emotional rejection by the people who were supposed to love her. Shira couldn't handle constant rejection as a child. She probably would have been able to if she didn't realize or understand it, but as a half-demon both she and Yuki matured more quickly, so they remember things from when they were an infant. Shira's breaking point was probably when her father tried to kill her.* (He knew what she was, and seeing her not only reminded him what was out there, but he also was afraid of her.) Either way, Yuki ended up taking over in an effort to protect Shira. This backfired because Shira never developed relationships with people who loved her.
The other reason Yuki is the demon half is that the world simply isn't ready for Shira to have the power of a Yuki-Onna without Yuki's influence.
As for the relationship between Yuki and Shira, they are essentially twins. Shira loves Yuki more than anyone, and is jealous of Yuki's relationships with other people. Shira despises Tasha in particular because Yuki and Tasha are sisters in a way that Shira wants. Shira is also afraid of being replaced because Shira Yuki bounced around foster homes a fair amount before ending up with the Johnsons. Basically, Shira hates Tasha for a lot of reasons, that are all psychological... Because Shira is super messed up. (I actually started writing a story about Shira and Tasha traveling to Japan. It's mainly about their developing relationship. It starts out with Shira trying to kill Tasha, again. Tasha's pretty much used to it.)
And if you think I'm making all of this up as I typed. You're half right. Well, a quarter right.
*Shira Yuki's father lost his mind because of his encounter with her mother. The Yuki-Onna probably would have killed him, but she realized that she was pregnant. Half breeds wouldn't survive on the mountain, so she kept the father alive so that he could take their daughter to his people when she was born.