Boku no Sekai()

Boku no Sekai

The fifth instalment of the Sekai Series.

An alleged prequel to "Kimi to Boku no Kowareta Sekai," when Byouinzaka Kuroneko and Hitsuuchi Samatoki were in middle school.

Note: Two of its chapters were released in Mephisto 2009 Vol. 2/3. Further chapters have yet to be released.
Nisio has mentioned Boku no Sekai in 2020 and 2021, saying how he has yet to finish Sekai Series and how he should, implying he may get around to finishing and releasing volume 5 soon.