Due to his ambiguities of his origin, there are theories written in novels, saying that Tenkai was a child of the 12th shogun, Yoshiharu ASHIKAGA, or that he was the same person as Mitsuhide AKECHI who killed Nobunaga ODA in the Honnoji Incident and committed suicide when he was attacked by natives who heisted lost samurai after the Battle of Yamasaki (it is usually based on a fact that there is a place named 'Akechidaira' in Nikko City where his graveyard exists).
その出自の曖昧さもあり、小説等で出てくる説として、天海が足利将軍家12代足利義晴の子という説や、本能寺の変で織田信長を討ち、山崎の戦いの後土民の落ち武者狩りに遭い自刃したとされる明智光秀と同一人物という説がある（墓所である日光市に「明智平」という場所があることなどが根拠に挙げられることが多い）。