Ranmaru ranks up when he reaches a 60% Link with a Riolu or a Lucario, while Nobunaga is in the same kingdom as Ranmaru.
Ranmaru's episode is Fate Born of Beauty. It is unlocked by completing the episode The Legend of Ransei and by having downloaded the episode from WiFi (when it was available) or using the password "2aL38Ek2Rx".
While Ranmaru is male, he appears in Ransei's Greatest Beauty episodes (including his own episode) because he his mistaken for being female. That is a reference to the fact that Ranmaru's gender is not obviously apparent from his appearance.
Ranmaru is based off of the real-life Mori Ranmaru of Japanese history.