Extra in Highschool of the Dead

"When I said I wanted more of Highschool of the Dead... I didn’t mean I wanted to be transported into the story!—A few erotic doujinshi and fan arts of the female leads would’ve been just fine!" Nach Smith, a 24-year-old—freelancer, part-time survival nerd, zombie apocalypse enthusiast, and full-time degenerate pervert—falls asleep in his bed after reading the discontinued manga Highschool of the Dead, only to wake up and find himself inside the story, right at the very beginning. Armed with his knowledge of the plot and an odd collection of skills, Nach will try to survive as an Extra in the story—saving other background characters he’d grown fond of while reading, accompanying the main characters, maybe even stealing a few female leads for himself, or accidentally taking on the protagonist’s role. Along the way, he’ll stumble through R18/Harem/Smut-like escapades—until he either reaches the manga’s unfinished part, wakes up from what he can only assume is a dream... or perhaps continues the story that never got its ending. — Description from novelcodex.org