Setsuna Mudo (無道 刹那 Mudou Setsuna) is the main character in the manga Angel Sanctuary. He is the reincarnation of the Organic Angel, Alexiel, but his life is even more complicated by the fact that he is in love with his younger sister, Sara Mudou. He is a typical rebel at school along with his friend Kira Sakuya, and eventually runs away with Sara to find happiness, but a tragic event occurs (Sara was killed) and Setsuna unleashes his anguish. He then journeys through Heaven and Hell in search of her, and gets entangled in the intrigue of the two realms.

With his cheerful personality, he managed to change many beings (angel, mortal or demon) he happened to meet. Kurai, the young princess of the Demons, is one of them. Though knowing that Setsuna loves Sara, Kurai nonetheless fell in love with the teen.

With Sakuya, it was quite another matter altogether. While Setsuna is Alexiel reincarnate, Kira was Lucifer, the ruler of the 9 Hells. When Sakuya was later destroyed by Rosiel and remade to Lucifer (His body, being mortal, is easy to dispose of. His soul, on the other hand, is nigh impossible to destroy.), Setsuna was distressed by the way Lucifer disregarded the friendship that once existed between Setsuna and Sakuya. But Lucifer helped Setsuna in his final fight against YHWH, thus betraying the fact that even he could not dismiss the close relationship the two once had.