Sekirei: Kanojo no Inai 365-nichi no Koto

Sekirei: About 365 Days Without a Girlfriend

セキレイ 彼女のいない365日のこと

The spin-off of Sekirei. Takes place 365 days before the return of Musubi. The tagline is "One year with his girlfriend gone. One year until she returns."

Note: The manga was published as the 19th volume of the Sekirei collection.