Daniel was a man of strong faith. He was among those taken into the king's service. He had his own beliefs about food and was not willing to consume the king's food which might have been against his principles.
Since I don't know the specific 'big brother daniel story', I can't really say who he is exactly. He could be just an ordinary boy in a family who has the role of a big brother.
In Armand and Daniel fanfiction, the writers usually play with the unique characteristics of the two characters. Armand's age - old wisdom and his sometimes - dark past can contrast with Daniel's more modern and perhaps naïve view. This contrast creates interesting dynamics. A lot of fanfiction might also explore their physical relationship, but not always in an overly sexual way. It could be about the closeness, the way they touch or look at each other that conveys their feelings.
It's a bit complicated. The Book of Daniel contains elements of both historical events and fictional elements used for religious and moral teachings. So, it can't be neatly classified as just historical fiction.