12 parts Ongoing Egyn didn't ever think he would become a father, but he wouldn't change or regret his choice ever.
When Rin and Yukio were born, The Demon King of Water, Egyn, took Rin. He broke the holy sword that sealed his power and raised him on in a safe haven for demon-human couples with the Nephilim children. Now, fifteen years later, his immediate younger half-brother, Samael has come to invite Rin to attend True Cross Academy.
Egyn is non too pleased.
Note, in my headcannon, the demon kings are ranked in terms of strength and age. Oldest to youngest.
Lucifer = Oldest; Tied for most powerful
Iblis = Second oldest; Third most powerful
Azazel = Third oldest; Fourth most powerful
Astaroth = Fourth Oldest; Fifth Weakest
Egyn = Middle Child; Second most powerful
Suzie/Mephisto = First youngest; Tied for most powerful
Amaimon = Second youngest; Tied for third weakest
Beelzebub = Third Youngest; Fourth Weakest
Rin = Youngest (By a few minutes); Tied for third Weakest
Yukio = (By a few minutes) Youngest; Weakest