Imouto sae Ireba Ii. Gaiden - Imouto ni sae Nareba Ii!
妹さえいればいい。外伝 妹にさえなればいい! / A Sister's All You Need. Side Story: All I Need is to Become His Sister! / Эх, вот бы и у меня была сестрёнка. Гайден: Эх, вот бы и мне стать сестрёнкой!
Rated 16+ Completed

Spinoff following Nayu as she endeavors to win Itsuki's affection by becoming his little sister by any means necessary.