Kusha is one of the twin sons of Lord Rama and Sita (the other being Lava). Born in the Forest after Sita had been banished from Ayodhya, they were educated and trained in military skills and were under the care of Sage Valmiki. When Rama performed the Ashvamedha Yagya, then they challenged their father (who was at this point unknown to them) to a duel by holding up the Yagya horse. When Rama found out their identity, he took them back to Ayodhya. Lava was given Dakshin Kosala and Kusha got Uttar Kosala.

It is Kusa who continues the line of Ikshvaku dynasty. He returns to Ayodhya soon after his installation at Kusavati, after the deity of Ayodhya complains of her abandoned state and appeals to him to return. He later fights and subdues the Naga king, Kumuda, and marries his sister Kumudvati, who ultimately gave birth to his heir Atithi. He was believed to be the ruler of a kingdom centered at Kasur in ancient times, and the present day Pakistani city still references him in name. Kusha was descended by his son King Atithi.