The heroine (Lili) is indeed a player. The kicker? Lili is one of the game's developers or...

The heroine (Lili) is indeed a player. The kicker? Lili is one of the game's developers or beta testers... and is a dude.
That said, there is no romantic tension between Lili and Rahnee. They actually develop a cute sibling relationship where they playfully bully the other! And yes, they do ally rather quickly. Lili did try to screw Rahnee over at first, but the situation turns hilarious very fast. This is one of those rare stories where the transmigrated 'villain' and 'heroine' actually help the other, and I LIVE for it.

Interestingly, it seems Lili, while he's aware that it's all a game, has lost some memories of his real life? Very unsure about this, take it with a grain of salt. But it also appears that Lili is unable to log out of the game as well.
So it appears that the game's relationship with the real world is complicated. Two NPCs in-game managed to teleport briefly to modern-day Korea in their dreams (I think one of them has a magical connection or something) and one of them actually sees the freaking advertisement for the game and is like, 'why are Lili and rahnee's face on this weird paper'. I should note that I call them NPCs because there's no proof of them being 'players' yet, though I won't be surprised if at least one of them did turn out to be an amnesiac one.

It is also revealed that Yona's (or Rahnee as she is in-game) presence in the story is VERY unusual because the Concubine Walkthrough game's release was delayed. Meaning there was no feasible way for her to have gotten into it because it was never released to the public. The issue? There was something wrong with the game's AI. Yeah... On top of that, guess what? Rahnee and Lili are NOT the only players!

Youngjung is actually a player as well! Somehow, he forgot all about his real/past life and became totally immersed in the role of Youngjung. As of the latest chapters (40+), he regains some (?) of his past life memories, and he knew Yona Lee! Looks like he had romantic feelings for her even then lol

So how this was found out was pretty cool. Basically, there was an assassination attempt on Rahnee's life, Youngjung saved her but got fatally wounded in the process, and we see him die... until he wakes up in this weird techno-field where Rahnee found herself in episode 7, with the big Rahnee avatar asking him if he wants to return to his last checkpoint with his teammates.

So you can guess what happened- suddenly EVERYONE (Rahnee, Lili, and Youngjung ofc) are transported 6 hours back in time. Rahnee and Lili don't immediately understand the situation outside of the fact time got reversed, so they look for Youngjung to save him from his previous fate. Thing is, he was searching for them as well and admits to having a very strange dream about dying for Rahnee. The lightbulbs in the girls' heads turn on, and they basically ask him to snap his fingers (this brings up the 'user menu' screen for all players). Youngjung does this, and a menu does pop up!! Pretty much confirming he IS a player. But as I said before, he doesn't remember being a player until chapter 40 or so.

From what is shown of his past life, he used to be a star football player in Yona's school until he lost a leg in an accident. I'm assuming Yona helped him recover from his ensuing depression. Also, Lili's real persona (the dude playing as her rn) looks to have offered real-Youngjung a spot in the game?
And finally about the emperor... I'm not sure if he's a player or an NPC. See, he can see Rahnee's user menu and Lili notes later on that NPCs can't see the menus, but when he snaps his fingers, his own menu doesn't pop up. Plus he appears to take his in-game backstory very seriously, relating a dead girl from his past to the current Rahnee (which is also why he's so taken with her now). Later on in the story, when Youngjung returns to a previous checkpoint, reversing time in the process, the world literally collapses around the NPCs. The emperor notices this and isn't shocked at all, saying it's happening faster than he expected.

My personal guess? The emperor might be the problematic 'AI' that delayed the release of the game. That, or a real developer of the game who's messing with the players for some reason lmao
You'll be happy to know there is, indeed, romance! More specifically, between Rahnee and the emperor.
I should again preface this by saying I can't read Korean for shit so I'm probably missing out on a ton of details, but the emperor (or 'Sunny' as she knows him) asks her out before she learns about his true identity. She gets into a bit of a conundrum about it because while she does genuinely like 'Sunny', she's also aware he's just an NPC (I think? She's definitely hesitant about the romance at first though). She eventually goes, 'heck it' and kisses him anyway.

A lot of chapters later, she gets into trouble with the empress dowager (Sunny's mother) (?). But before the situation escalates, the emperor arrives (and stops his mom I think) and everyone quickly gets on their knees and bows (head-to-floor). Because of the position, Rahnee doesn't see his face immediately, though she recognizes the voice as Sunny's. She looks up and finally learns that Sunny is true, the emperor, and is horrified to see the usually cheery and cute face she associates with Sunny replaced with the cold, emotionless glare of the emperor.

So some dialogue happens and the emperor ushers everyone out of the room except Rahnee. Now she's obviously afraid because the emperor is the number one cause of Rahnee's death in the original game. But now that they're alone, the emperor can show his true emotions and hugs Rahnee, presumably as an apology for the stress he caused her earlier. It's all good between them after, because in less than 10 panels Rahnee kicks the emperor in the gut, aka the usual relationship lol. Of course, this means that Youngjung lost his side of the bet (if Rahnee remains in the dark about the emperor's identity, they both get to leave) and is distressed about it. I think.

Anyway, Youngjung is definitely the second male lead. After regaining his real life's memories (and feelings for Yona maybe) he suddenly gets this dark, scheming look and approaches the emperor with what I assume to be hidden motives. I'm not sure if Youngjung knows that Rahnee IS Yona, though.

So in a nutshell, it looks like the emperor genuinely likes Rahnee. But again, he has related her to a girl in his past who also called him 'Sunny', so whether it's genuine love for her personality or just because she resembles this dead girl, I'm not too sure. I will say I'm personally on team Youngjung hehe

credits to Voodoohater from novel updates spoiler forum