En was the Great Child of Primas, the first of the Twelve Worlds. Commanding significant power and respect, he stood as the strongest among the Great Children and a central figure of admiration. His duty was protecting the land from chaos-spawning creatures known as Trouble Monsters.
En encountered Bucky, a young boy determined to conquer the world. Recognizing Bucky's potential and resolve, En chose him as his successor. He entrusted Bucky with his Spirit, Jibac, and the G.C. Watch, passing the mantle of Great Child of Primas before departing from his active role.
Despite his departure, En reappeared during moments of extreme peril in Bucky's journey across the Twelve Worlds. He maintained a connection to the Needle Tower in World Zero, revealing it as his intended destination. Existing in a transitional state—neither fully child nor adult—this place held significance as the domain disrupting the balance across the worlds.
En shared a history with the Princess of the Needle Tower, his sister. They originated from another planet destroyed by the antagonist Slash. After an attack on their spaceship, the Needle Tower, the Princess was paralyzed and placed into cryogenic sleep. En survived, becoming the Great Child of Primas before selecting Bucky.
Later, En intervened when the Princess, awakened from cryostasis, became possessed by Slash using the Poison of Awakening. She battled both En and Bucky. During this conflict, En employed the Prayer of Heaven technique, purging Slash's control from his sister at a critical moment.
Following Slash's defeat, En and the Princess bid farewell to Bucky and the other Great Children. They departed the Twelve Worlds together aboard their spaceship, the Needle Tower, returning to their home planet. The Princess's sword, left behind, was later wielded by Bucky in the final battle against Slash.