Countdown 7 Days

Onigawara Hanasuke is dead. He only realized that when Mitamura, a dark haired glasses-wearing guy from another world, has confirmed it. The truth saddened Onigawa, but Mitamura offers to teach him how to revive himself. However, in exchange, he needs to help Mitamura find someone... and the time limit's seven days.
(Source: M-U)
Note: Released as three separate oneshots in Comic Blade Avarus.