It's pretty sad that the final ending is... not fully finished and most of the story is still in mistery... or hiding in the game for others to dig.
*[Spolier alert]*
I kind of get the direction which sukeban games is heading. A generated or imagined dimension (forgive my poor vocabulary) which Rune and Shara trapped. They are separated for some reason and Shara blamed this to Rune. The comment from @calf may have explained the story well with reliable reasons. But I wish I can see a full picture on Sukeban game's next game... as an easter egg or something.
Clearly creators want to put more stories between the last two levels, but unfortunately ran out of time - just like any other Game Jam game lol. But so glad to have a polished level 4, with two characters rely on each other and show us their love. And the last puzzle is so cool - using the previous puzzle as hint. Although yes it requires players to remember the previous answers... it is kinda not very friendly.
And it is obviously Sukeban game trying to make the replay different from the first time. It is unfortunate.
*[Walkthrough]*... if anyone still need
1. Hint: Shara is always 2 years younger than Rune, no matter what you enter at the beginning. And there is a date only occurs each four years. Because Shara's birth year is depending on your input, so her birth year may not be an actual leap year. You can check @calf 's answer and it is more obvious. Answer: [First Year](https://-1996-) [Second Year](https://-1998-) [Month](https://-2-) [Date](https://-29-)
2. Hint: Click everything. And again. To be honest I do not fully understand what is going on but just cooking with some liquid... Answer: [Hover here](https://-360-)
3. Hint: Set up the defibrillator and use it. Double check the mirror - they draw the CG for some reason. Answer: [Hover here](https://-363-)
4. Hint: What is the symbol of infinity? Answer: [Hover here](https://-8-)
5. Hint: Click everywhere. And again. Do a little math of how much gas they need. Answer: [Gas Type](https://-91-) [gallon](https://-5-)
6. Those question are referred to the previous answers. A little poet hiding some math there. Thanks for @jusw85 , your spolier helppe me. Here I will repeat the answer from them. Answer: [First lock](https://-5-) [Second lock](https://-90-) [Third lock](https://-131-) [Forth lock](https://-2168-)