The story begins eight months in the future of the main story, with Nana and Kaoru in the summer of their final year of high school. It uses a different supporting cast and a different location for the setting, and as such it doesn't really spoil anything from the main series.
Denpa
Related works
From Denpa:
Rei Kiriyama is a child prodigy. Rei Kiriyama is also an orphan who lives alone in an empty apartment. Rei Kiriyama is a teen working in an adult world. Life is complicated for Rei. He's an up-and-coming shogi (Japanese chess) player on the verge of turning pro but he has no home life or much of a life period outside his board game but thankfully with the help of…