wait she comes back?? then what does that mean for the resolution of the story in the anime? is it ok for her to leave the real world again?
So, just a little background info. Both the OP and the ED were written by the director and are key plot elements.
If you've ever listed to the full ED song lyrics it's actually a full time line of the series. What happens before the anime started, what happens during it, and what happens after. Someone explained in full here, https://i.imgur.com/7Ifx5mB.png
You'll notice that when Akane and Rikka are together in the time line the song lyrics say "connected" and that happens at the end of the song.
You'll also notice that the song mentions Rikka breathing. The last few lines of the final voice drama is Akane talking about her's and Rikka's breath. You can download the voice drama here https://nyaa.si/view/1131239
In the last episode of the anime Rikka wishes to be with Akane forever, and that her wish will never come true. Because she wants Akane to leave so she can learn to love herself and face society in a healthy way. But, this doesn't mean they can never see each other again, just that Akane has to grow up first. Which the song lyrics suggests that she does. As well as the OP song's live action music video, which is real life Akane, and we see her start making friends.
It's actually kind of crazy how much attention to the little details went into Gridman. Over a year before the anime aired the director wrote a yuri web novel. It stars Akane's two classmate friends. Ramo and Ako. And they show up in Gridman's anime as well. The novel mentions an out or services water fountain and that shows up in the Gridman anime as well.