This is the description of the person of Daniel given by Josephus, including the fact that he was related to the Judean royal family
Antiquities of the Jews 10:10:1
1. But now Nebuchadnezzar, king of Babylon, took some of the most noble of the Jews that
were children, and the kinsmen of Zedekiah their king, such as were remarkable for their beauty of
their bodies and comeliness of their countenances, and delivered them into the hands of tutors, and
to the improvement to be made by them. He also made some of them eunuchs; which course he took
also with those of other nations whom he had taken in the flower of their age and afforded them their
diet from his own table, and had them instructed in the institutes of the country, and taught the
learning of the Chaldeans; and they had now exercised themselves sufficiently in that wisdom which
he had ordered they should apply themselves to. Now among these there were four of the family of
Zedekiah, of most excellent dispositions, the one of whom was called Daniel, another was called
Ananias: and the king of Babylon changed their names, and commanded that they should make use
of other names. Daniel he called Baltasar; Ananias, Shadrach; Misael, Meshach; Azarias, Abednego.
These the king had in esteem, and continued to love, because of the very excellent temper there were
of, and because of their application to learning, and the progress they had made in wisdom.