Sage Valmiki is the author of the Ramayana which he wrote in a poetic manner. There is not a single stanza in Ramayana, where he has not used a figure of speech. He is called Adi Kavi and the Ramayana, Adi Kavya.
Valmiki was a hunter by birth, who sustained his family by killing animals. One day Narada Muni was passing through the area and met him. He talked to Valmiki and asked him whether his family members, who shared the food that he took home, were willing to share his sin of hunting and killing the animals. Valmiki thought they would but decided to ask his family in the afternoon.
When he went home, he asked them and was surprised at the answer: none of them were willing to do so. This upset Valmiki greatly and he decided to give up his sinful ways and search for a better life.
He asked Narada Muni for advice, and Narada advised him to chant the name of Rama. After some time, in his absorption of meditation, he did not notice that an anthill was built round him.
Valmiki became a sage and a poet. He was searching for a suitable hero whose story he wanted to write in a poetic form. In his pursuits, one day he saw a hunter killing one among the bird couple, which resulted in the heartrending cry of the other. This touched his heart and soon he met Sage Narada again. Narada requested him to write the story of Rama which revolves around the Lord's marital separation and thus the Ramayana was written.