i support the theory of Ashura being Fai's adoptive father,because this kind of relationship is based on pure confidence,affection and complicity,so according to the fact that Fai seems totally broken by his past,it may means that he has really been betrayed by his most important person,the one he has always admired through his child's eyes,and who has given him everything he had like happiness comfort and a shelterÂ
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Fai was maybe a abandoned child who was rescued by Ashura,and at the beginning we can suppose that Ashura was a very caring father,but that when he has discovered the magical power of Fai he has changed and has decided to exploit it for his own purposeÂ
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i don't think that Ashura was evil,he has certainly been in contact with a devil aura which has bewitched him,and so that's why Fai was forced to seal him to put an end to his madnessÂ