Harun (peace be upon him) was the brother of Musa. When Musa was commissioned, he prayed to Allah to appoint Harun as a helper and to strengthen him through his brother, for Harun was more eloquent in speech (20:29-32, 28:34). Allah answered his prayer and made Harun a prophet, saying, "We will strengthen your arm through your brother" (28:35, 19:53).
The two brothers went together to Pharaoh, commanded to speak to him gently, in the hope that he might take heed or fear Allah (20:42-44). Harun stood with Musa through the confrontation with Pharaoh and the delivery of the message.
When Musa departed to Mount Sinai, he left Harun in charge of the people, telling him to set things right and not to follow the way of the corrupters (7:142). In Musa's absence the people fell into worshipping the calf, and Harun tried to restrain them, warning that they were only being tested and that their Lord was the Most Merciful, but they would not listen until Musa returned (20:90-94).
Musa, in his zeal, seized his brother, but Harun explained that he had feared causing division among the Children of Israel; Musa then prayed for forgiveness for them both (20:92-94, 7:150-151). Allah counts Harun among the favoured and guided prophets (37:114-122, 6:84).