His name means 'Horse-friend'. He was son of Marhari, the leader of the Northmen who fought with the Gondorians during the Battle of the Plains. After his father’s death, he led those of his people who remained to the Vales of Anduin, after many of the Northmen had been divided between the lands of Dale and Gondor. These people who remained under Marhwini, were the beginnings of the Éothéod who would one day become the Rohirrim.

Marhwini and the Éothéod were allies of Gondor during their war with the Wainriders. He sent word to King Calimehtar of Gondor that there were some Northmen who were planning to revolt against the Wainriders who had taken their lands during the Battle of the Plains. The king and Marhwini planned an attack and it was executed to coincide with the Northmen’s revolt. The Wainriders were defeated and they were quiet for a time. Gondor enjoyed many years of peace and Marhwini retired his folk back to their lands in the Vales of Anduin. The alliance between the Éothéod and Gondor had begun.

To read more about Marhwini and the Éothéod, see the link above to the Middle-earth article called 'A History of The Rohirrim'.