Vālmīki is the sage who has immortalised himself and his work, the Rāmāyaṇa.
According to one version, he was a prajāpati, a son of Pracetas.
According to another version (Adhyatma Rāmāyaṇa, Ayodhyākāṇḍa 6.57-78) he was a brāhmaṇa by birth, but was brought up by robbers. Once, the Saptarṣis (the Seven Sages) came to his forest and were accosted by him. Contact with these holy sages converted him to pursue a life of devotion to Rāma. Since he sat in meditation for many years, an anthill grew over him. When the sages—on their return journey—found him in that condition they poured water over the anthill, released him and blessed him with a new name ‘Vālmīki’ (one who emerged out of an anthill).
Vālmīki established his own āśrama and started living there. One day when he came to the river Tamasā for bath, he saw a hunter kill a male bird and heard the piteous wailings of the female. Immediately he cursed the hunter, which curse emerged from his mouth in the form of a śloka (a verse in the anuṣṭubh metre). Wondering at this phenomenon he returned to his hut. Then Brahmā, the creator appeared before him and commanded him to compose the detailed life-story of Rāma for which the basic material was given by the sage Nārada. Thus came into existence the great epic Rāmāyaṇa.
When Rāma banished Sītā due to public criticism, Lakṣmaṇa left her near the hermitage of Vālmīki. She was pregnant at that time. She delivered twins who were christened by Vālmīki as Lava and Kuśa. It was Vālmīkī who brought them up, educated them and also taught them the Rāmāyaṇa with music. Later he took Sītā and the children to Rāma and offered them back to him. While Sītā entered the womb of Mother Earth, the sons were accepted by Rāma.