Nisien, a character in Welsh mythology, is depicted as the twin brother of Efnysien and the son of Penarddun and Euroswydd. In the Second Branch of the Mabinogi, Nisien is portrayed as the opposite of his brother Efnisien in terms of personality. While Efnisien is vindictive and destructive, Nisien is described as tranquil and generous.

Despite their differing natures, both Nisien and Efnisien play significant roles in the events of the Mabinogi. Their actions ultimately lead to a series of tragic events, including the mistreatment of Branwen, the war between Britain and Ireland, and the deaths of many characters.

Nisien’s contrasting temperament adds depth to the narrative, highlighting the complexity of human nature and the dynamics within the family depicted in Welsh mythology.