Character Description: Reika Mishima
Mishima Reika is another unusual character in the story who seems to be the one in Kamina's paintings. She is instrumental in Kamina's relationship with RahXephon. However, who she is, or even if she really exists, still remains largely unknown.
However, if you watch all the way to the end of RahXephon, beyond the credits, it becomes more clear who Mishima Reika really is... she does exist, and is Haruka Shitow when she was young. For some complicated reasons (expounded more thoroughly in the series) her name was changed; and of course, when Ayato was brainwashed, he didn't remember who she is.
Still, it is correct to say that we do not know exactly who is the being (Ixtli) who appears in her form throughout much of the series.