Delusion10
Probably save Arche and her little sisters. Sisters because even in it's lore he would try to help them and Arche because he could use her (howewer how it's sound) as test to how people of new world could lvl up if nutricien by Nazarick and having sisters would destroys plan to run away. In another case you could do as in web novel to make her Shalltear s*x tox who teach later Ainz how to dance (this really happen in web novel), in meantime Cinitras as Ordan or himself take care of her little sisters. Maybe even as some strange parental figure