Yui is killed by Light Yagami.
Light has somehow opened the book in episode 12 and when he does, he sees Yui becoming bad. Because of that, he will kill her, knowing that she will cause his New World to an end. However, he decided to wait until she is out of the book.
By the time Yui is out of the book, Light will make an attempt to kill her, but has decided to wait until he finds proof that she is bad. When he saw her sending Suboshi after Miaka, Light has begin writing in the Death Note her name and the details that is going to happen in episode 50.
The Byakko series, assuming it gets created, will be Lighter and Softer than both previous FY series
Yuu Watase has said that if she does a Byakko series, it would be a completely different beast from both FY and FY:GK, and what we've seen of Tatara, Subaru, and Tokaki, the series seems more set up to be a romance/comedy. At least to me.
Yui has Split Personality.
This personality is created after being raped. It becomes evil because of Nakago convincing her. Therefore, for the entire time, the bad personality takes over completely while the good personality, the real one, tries to get her body back since she knew Miaka didn't betray her.
In episode 50, the real personality finally took over, and uses the final wish in order to take the dark personality with her. Therefore, when she's returned in the next episode, it is assumed that the latter is no more.
Chichiri is a Time Lord.
His hat is his TARDIS.
Taiitsukun is a Time Lord.
Mt. Taikyoku is her TARDIS, and Chichiri is her Companion.