That's part of the mystery.
Okay, non-vague answer: it works both ways. The chosen can order the legendary in battle and it will follow such orders most of the time. But the legendary acts as a mentor figure too, in which case it is also the master. Divine interprets the legendaries to be like the gods and goddesses of Japanese tradition. They are immensely powerful, practically immortal, but they still have failings and mortals can still outwit them from time to time.
But I'm always open to experiments in writing and interpretation.