There's some good material in the Guide on this.
Tosti Runfriend's account (p732) says that "While [in the Sky realm], Kallyr and her companions befriended the Star Dragon so that she could defeat the Empire and free her people. This was all in preparation for a far more dangerous magical ritual Kallyr had planned." IIRC, Gathering Thunder's story line has this occur as well, though off-stage from the PCs, and after they leave the Sky realm. Later the same account says that "even Orlaront Dragonfriend did not know [the dragon]. So he was new, brought there for that moment by the magic of