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6.   ARE GODS AND DEMONS RELATED?

Yes, at least to some extent. As in Norse mythology from which AMG is
derived, the good guys and the bad guys are not that different. They
can reproduce with each other, and apparently as children gods and
demons play with and can even be close friends with each other, as
Urd
and Mara did (V10 Ch60). It is also possible for them to switch sides
(V21 Ch135), though this seems to be very rare if it's ever happened
at all.

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7.   WHY DO GODS AND DEMONS NEVER KILL EACH OTHER?

Killing a goddess or demon is expressly forbidden. Sealing for 500
years, eternity, etc. is allowable, but killing is absolutely not.
The
Doublet System enforces this rule.

Note that there seems to be no rule against killing mortals or
magical/demonic creatures. Urd says she will destroy the anki in V13
Ch75 and Velsper is prepared to kill Keiichi in V16 Ch97.

7A.  WHAT IS THE DOUBLET SYSTEM?

The Doublet System ties the life force of a goddess to that of a
demon
so if one is killed, the other will die also. At an age of
approximately 8 (Belldandy and Velsper appear to be about that age in
V17 Ch99), a child from each side is selected to undergo the
ceremony.
The criteria for selection is unclear, though magical ability seems
to
be a large part of it. Each side knows who they have selected, but
not
who the other side has selected. After the young goddess and demon
have completed the ceremony, their memories of the event are sealed
so
even they don't know who their doublet is.

It isn't clear how the Doublet System was instituted or how one side
or the other is kept from cheating. However, it works very well.