One of my favorite mini-genres of doujinshi is the "what if thing was made into a fighting game?" Not everyone has the talent of a French-Bread, so some folks just make little doujin publications with illustrations and imaginary movelists. I've seen folks do this for Fire Emblem, Love Live, Dragon Quest, and a few other properties... and this, Sega Gal's Star, a copy-bon from creator Edoguro and circle Michinoku Hiroba.
(A copy-bon, as the name suggests, is a doujin printed through a photocopier and stapled by hand. A fair amount of doujin is made this way, as it's cheap and works well for stuff you expect to have very limited distribution. Places like 7-11 even have templates for easy doujin printing these days.)
As you might surmise from the cover, Sega Gal's Star is a 28-page book styled like the manual of a fictional Megadrive game, explaining the game's controls and showing special moves for all the the game's characters, which happen to be female characters from Sega games. Six are shown, and a lot of them are pretty deep cuts: Besides Phantasy Star's Alis, Phantasy Star II's Nei, PSO's Rico and Streets of Rage's Blaze, there's also Papli from Girl's Garden and Kurumi from Ninja Princess. At the end of the book are four pages presented "company lineup" style of a whole mess of Sega women who would likely be potential candidates for "inclusion," complete with handwritten notes.
If you asked me to give an example of "Why do you love doujin culture so much?", I'd probably point to this. It's the kind of lovingly produced, wild-dream fan project that brings me joy just knowing somebody had the passion to make it exist.
