Who came to help the messenger sent to Daniel when the prince of Persia opposed him?

Answer: Michael

The messenger was opposed by the prince of the kingdom of Persia until Michael, one of the chief princes, came to help him.

“However, the prince of the kingdom of Persia opposed me for twenty-one days. Then Michael, one of the chief princes, came to help me, for I had been left there with the kings of Persia.”— Daniel 10:13 (BSB)