Ah, well you'll have to wait until I introduce Mika as a full character (probably coming soon). As to how she unlocks, well that's kind of a spoiler, but I'll give you a hint if you're willing to spoil it a bit.
Like I say, though, that's for a future update.
Yes, it's deliberately designed to be fairly short, but each time you get an ending you'll unlock bonuses so it's much easier to unlock them in future. Meanwhile there will be other new features that start to unlock on new games once you've completed endings (once I've got to the point of including those).