On another forum someone who has the art book posted some scans and one of the pages features the cat and the robed, skeleton man. Someone gave their translation of the page:
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The master of the old castle is the one who commanded the ancient Tiamuts to build the Avenheim castle. He is the "Sage" of 1000 years ago that was told in the story, and by Transoul, he has lived up to now. Intellegent, powerful in magic, curious, loves to be alone, etc. After a long time, he reached the question "Why did the Gods create humans?". He creates Avenheim, a huge testing facility, and gathers lots of people to observe them. And so, like he thought, the humans would repeat their history of fighting over political power and greed, and he has been in the castle observing this for 1000 years.
The page is this one: (Possible spoilers?)
http://nihon.kaisho.org/Random/Knights%20i...0Scan%20121.jpg