Specials included in the DVD/BD volumes of the anime. They feature original stories written by the series' mangaka, ONE
The first special is titled "Shinobiyori-sugiru Kage" and takes place after Saitama's fight with Mosquito Girl, and it follows Genos as he is persistently and suspiciously prowling after Saitama trying to find out the secret of his strength. Additionally, a mysterious person appears who has a "certain reason" to hold a grudge against Saitama.
Manga