I always found the big twist in 'Re:Zero − Starting Life in Another World' less about Regulus's own backstory and more about how his authority works. He goes on and on about 'love' and being a 'husband,' but the real kicker is the nature of his 'stillness.' It's not just defensive invincibility. His authority, 'Lion's Heart,' actually stops time for anything he considers 'his possession' – which, horrifyingly, he extends to the very air around you, or even the space your body occupies. So when he claims a wife, he's not just being creepy; he's literally making her a 'possession' frozen in a single moment, unable to age or change. That's why he can't be harmed by anything. The plot twist is the cruel irony: the man who screams about his rights and love is fundamentally incapable of any real connection because he's frozen his entire world, including himself, in a state of absolute, sterile stasis. It's a brutal deconstruction of selfishness disguised as principle.
Honestly, the anime adaptation made it clearer with the visual of the frozen crystals. The moment you realize he isn't just tough, but that he's cheating on a conceptual level, recontextualizes the whole fight. It turns a powerful villain into a profoundly pathetic one. Subaru had to win not by overpowering him, but by out-thinking the very rules of the authority—using Regulus's own warped logic against him by having everyone 'return' his 'possessions' so the authority's protection would lapse. The twist is in the mechanics, and it's brilliantly messed up.