User blog comment:B214/High School DxD Hero Discussion/@comment-24866895-20180424173252/@comment-28884330-20180424204121

That's not a new design for Serafall, it was just a one time thing she also did in Volume 9, wearing a yukata during the meeting. It's not her average look, which still is the Mahou Shoujou outfit. The only permanent change will be her eyes are now blue to match the LN.