OHMAHGAH! He can load saves from multiples points in the past! Unlike Subaru, his load point can go further than the 'last saved point'. Crying fake tears right now, got to see Rubia again, this time-! This time, he has to protect her!
...it seems they're going with a visual novel approach. Where you have to clear multiple routes to understand what's going on and only then you'll be able to reach the true ending...
was his checkpoint the partner he have at the time? if none active it goes to the default Rubia? or maybe scenario based? like if he reached a clear point and he died after it revert back to Rubia?
man they keep introducing more dangerous threats left and right. At some point he's probably gonna be killed by a Godlike entity.
I assume the checkpoint is if there's a girl he's fond with in his party. With him going separate ways with the thief girl, he returns to the default checkpoint instead.