User blog comment:B214/Question and Answers 6/@comment-121.44.63.98-20171028032432/@comment-29963364-20171028033320

Basically, she is

"a dignified lady with a strict demeanor, but she is also a kind-hearted woman who cares for her grandaughter's happiness and well-being. While she is strict, she is open-minded enough to accept Rossweisse's decision to become a Devil (after giving her a stern lecture about making such a brash decision), and choosing Issei as her lover."

At least, that's what I got from reading vol.17.