Jade

Just to help, Cirus wasn't from a parallel world, but these in fact exist. when he was hit by a car the first time, the other Cirus (dress in black, black cirus) show him his world, a parallel world when he a phu were together since high-school. present cirus though it was a dream and that's why he lied, but they were memories. this has happened for who knows how much, because the reason black cirus did talk with phu in high-school, was because he also had a "dream" with the memories of other cirus, the adult one in a offices from ep. the same thing happened with him when he was stabbed and so and so. black cirus told present cirus that it will be his turn to show his memories to another cirus. and that kind of happens in the cirus, when ponytail cirus came. the special episode will be about that cirus, but basically the reason why ponytail cirus is going to talk to his phu is thanks to present cirus memories/life.
it was confusing just because present cirus could never remember the white room and seeing black cirus. for him it was a confusing dream.
hope this helps! :)

Lee Seung Gi? Just in 2021 in dramas he was on Mouse, A lot of tv shows like new world and busted. He has been very active, you can see it in his profile.

I actually love this drama. Many people don't get the end, but I'll explain you: Yes, they actually die. The scene where they come back from the ocean it's that they're both in heaven now, so that's why she is no longer in vegetative state. And then, when they get marry, years have passed, and that's why all their friends are there with them, even the guy who died in the accident: all of them have died, that's why they say: "It's been a while since the last time we were all together."