Huh? no way, really? a Ruri book? awesome, thank you very much, this needs to happen in the actual manga, she must be part of the harem. Edit: It's official, Ruri for Harem Queen, she must be the promise girl of Raku.
Very awesome that someone finally made a doujin entirely about Ruri, THE BEST supporting character and really deserving to be a heroine. It would be a really really unexpected in a good way, twist that Ruri is the promist girl as pscicomenace said.