Silen is a female demon of the prehistoric Demon Tribe, accidentally freed from underground during a battle between Mazinger Z and Doctor Hell's Mechanical Beasts. She possesses a deceptively beautiful humanoid form concealing her true monstrous nature and can fly using wings. As one of the first demons revived in the modern era, her primary allegiance is to the Demon Tribe's leader, General Zannin, serving as a key soldier. After being freed, she journeyed to the Himalayas, where Doctor Hell located her frozen tribe. He awakened them and proposed an alliance: the demons would help eliminate their enemy Devilman in exchange for Doctor Hell's aid against Mazinger Z. Silen participated in this pact, unaware Devilman witnessed their agreement.

As part of the alliance's operations, Silen was dispatched alongside the amorphous demon Bugo to sabotage the development of Mazinger Z's Jet Scrander flight system. During this mission, she damaged the equipment and took Shiro Kabuto and Sayaka Yumi hostage, subsequently dropping them from a great height. Devilman intervened to rescue them, revealing his presence to Mazinger Z's pilot, Koji Kabuto. Silen met her demise during the ensuing battle above the clouds when Koji, piloting the repaired Mazinger Z equipped with the Jet Scrander, speared her head with a massive icicle. This character appears exclusively in the 1973 film.