Every update is guaranteed to give every main character at least 1 story event. v0.16 is actually the exception where I state in the post that it is a balance update. Main characters like Lin and Ashley will get no story this update as they have the most, and other mains will get extra story. This has been requested for a long time so I figured I would dedicate at least one update to some balancing.
Jin's only content was sex in the dungeon, now she has story and is getting a lot more content. As this game is in development, you will experience the game in a way it was never intended to be experienced, since the game is intended to be experienced in it's completed form. This means there will be awkward moments like Jin feeling like shit for a long time in your game, when in reality, I just haven't developed her next event yet and would be over much quicker.
I am not sure if by "master/slave relationship" you mean an abusive power dynamic, that remains locked to slave route content within Nia and Sylvia's story.
Side characters will also be getting story of course, and there are more plans as well. The post above lists the new mechanic coming in v0.16, which is an optional expansion to the early game.