Not a Tamagotchi but that's a fun guess. Each Unlockable needs a base thing which you then add to with other Unlockables... this 90s favourite was introduced in Tape 2 of the game.
I just posted a new Thought about this. Unfortunately, I have to delay Season 2 until next month. The complicated coding stuff is mostly done, the time consuming part is creating the assets needed. Sorry for the delay!
Wait, I didn't even notice the birthday month dates haha. That's a very cool coincidence actually! But of course, the Gems are based on the girls and the new Unlockables are also relevant to each girl...