I think that by them fighting is part of them being able to beat there father in the future. Because both of them will be need to stop satan from recking Havok in the world.
I have read this manga like 5 times already and it still makes mi cry and happy at the same time. I love this authors work most of the mangas they write hit the heart strings all time.
I love this manga so much I hope that they make a season 2 soon I need more of this.