The Usenet manga Glossary, compiled by Iain Sinclair has this information
on blood types:


Ketsuekigata 血液型

"Blood type". In Japanese pop culture, blood type is thought to be related
to personality. This belief became popular in the early '80s. Profiles of
manga artists or characters from manga will often include blood type along
with other statistics like age and place of birth. A very rough guide to
blood types: 

A    nervous, introverted, honest, loyal
B    outgoing, optimistic, adventurous
AB proud, diplomatic, discriminating
O   workaholic, insecure, emotional

So some guesses:
A    nervous, introverted, honest, loyal (Keiichi)
B    outgoing, optimistic, adventurous (Definitly Urd!)
AB proud, diplomatic, discriminating (Belldandy, although her pride is not
often on display)
O   workaholic, insecure, emotional (Skuld)



Email adress of nervous, introverted, honest, loyal Bas Suverkropp:
basjanni@worldonline.nl