With the spin-offs, they usually don't release them the characters at once. In the case of Drowned Into the Night, Munenori, Goemon and Hanzo were available for a certain period of time. Once that time ended, Saizo and Musashi were released until May the 9th.
I think only the spin-offs that you buy with Ninja Coins and that actually say "Free from Rice Balls", don't require Power (the equivalent of Love Passes) to read. All other spin-offs do.
Feel free to ask if you have more questions~ :3