Hellsing
From Jenny Everywhere Wiki
The Man With No Name once read an issue of a manga called Hellsing. Its tagline was “Holy sh*t, vampires!”. He was irritated by a plot development he found nonsensical, wherein which a major male character transformed himself into a twelve-year-old girl. (COMIC: Jenny Everywhere Rules 2)
Behind the Scenes
In our world the manga Hellsing ran from 1997 to 2008 and was collected in ten tankōbon volumes, with various minor spin-off stories. The volume in the comic appears to be one of the latter, the prequel Hellsing: The Dawn, in which the described event happens.