Mahou Shoujo Nante Mou Ii Desu Kara.

Chapter 19.5: Volume 1 Extras
Chapter 19
Chapter 20
Chapter 19
Chapter 20
The story revolves around Yuzuka Hanami, who is approached by a mysterious creature that tells her she has the makings of becoming a magical girl. However, Yuzuka's magical form has her wearing a swimsuit!