User blog comment:SpookyBoogie27/Light Novel Spoilers/@comment-33447191-20130131105522/@comment-33447191-20130201062012

Okay, so first thing's first ...

God is Dead. He perished in the Great War along with the Original Satans. Michael, however, has differing opinions. Unlike those who desired war and dominance over one another, he wanted peace and prosperity. So much in fact, he gave Issei Ascalon as a sign of good intention, apologized to those felt The System had wronged, and persisted in the creation of the treaty between the Three Factions along with the other leaders. Also like you said before about the Issei being the Sekiryuutei and all but also because I feel that Irina has actual, passionate feelings for him being a childhood friend and all. Rather you know how she screamed for him during his match with Sairaorg ...

[I also like him! He’s really lecherous and he always thinks about ecchi things…… But he’s always burning more than anyone. He doesn’t give up. He works hard. I know that he is a person who can fight for those he loves! Everyone knows that as well right!?]

Finally about their child being a 1/3 Angel, Devil, and Dragon, I honestly see no problem. The entire point was so that Irina would not fall, although there have been hints that suggest  that had Issei asked, she would have "fallen" to be with him.