
With the clues we've got so far:
1. This is their second life. Ido has some recollection of the past life. Sejin is unaware, but his subconscious knows.
2. They were partners in their past life, Ido seemingly distant, possibly abusive/hostile to some degree? Sejin's family situation was even more rocky. They weren't happy.
3. ... Did Ido shoot Sejin????? Is that why the gun is mentioned???????? It needs to be fired now that it was mentioned, but maybe it already has been fired..?
... Or Sejin did shoot himself? My bets are on that.
Ido then had enough regrets to get a second chance

1.not just some memories he remembers everything
2. Ido loves him but clouded with lies by his brother, and Sejin really did fvck his company a lil bit by his father's order so they become distant, still ido loves him. this is why on the present life his brother is missing Ido took care of him ahead of time.
3. any more than this i will spoil the fun for you but great speculations , i just said the facts where the events already happened.
we are slowly getting into the angst part of the story.

there's no spoiler tag on replies but i hope mangago wont cut my comment, if this goes through please dont click my comment its a spoiler
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No Ido didn't shot him with the gun, Sejin killed himself with Ido's gun.. Sejin got pregnant but Ido's brother told Ido that it was him that got Sejin pregnant because he raped him multiple times, Ido being a stupid that he is almost believed his brother, Still in love with Sejin he confined Sejin at their house, when in fact the baby Sejin was carrying is Ido's child. Ido realized this a little too late he found Sejin's body with his gun beside him..
Sorry if my explanation is messy im driving lol, i tend to forget things so might as well reply while stuck on moderate traffic.

The official feels a lot more soulless, even from the re-name ... why not just keep 'my guildmate next door'? It's cute.
The lack of the small details and care? From the missing hearts replacing the Os in the couple chat, or the lack of slang ...
Official releases need to be more careful about quality, because if they're worse than what fanlations can do, then why buy the official releases when the grass is actually greener and free on the other side? Of course, support the author, the little they'll get from from their slice of the overpriced international release that the publisher put.
I wouldn't pay ~2 usd for this every week, even if I didn't know about mgg and how good the scanlation was for this particular series.
It just feels oh-so-much more flat.

[read this for a better translation of the end of chapter 50]
"My sister" should be "my noona", talking about an upper-classman he knew from working on a group project (per the novel)
Also if you don't like waiting for the fantastic scanlators that are the cat cafe team, check up on the translations of the novel. It's on novelupdates, same title as here. Show that translater some support, they're doing good work.
Because the final dialogue between the mc and ml was akin to a rat's ass here, here's the ACTUAL, beautifully awkward conversation, taken from the mentioned fan-translated novel:
“I haven’t known you for long, so it’s natural for me to find things unexpected when I see sides of you I didn’t know about.”
“Is that… a compliment?”
He didn’t seem to understand.
“What I mean is… You might actually be talkative and different from the personality I know. But that doesn’t mean the side of you I’m seeing now isn’t real. Are you acting right now?”
“No.”
“Then it doesn’t matter.”
The guy next door might still be shy around him. But just because that shyness fades over time doesn’t mean Yoon Ji-gu will suddenly become someone else. Yeo-woon was not sure if this was the answer Yoon Ji-gu was hoping for, but this was what he thought.
“They’re both you.”
“…”
It wasn’t a particularly profound statement, but Ji-gu’s eyes widened in surprise. Their gazes intertwined in the air. On a humid summer night, the rhythmic jazz music flowing out each time the cafe door opened lingered in their ears.
Yoon Ji-gu continued to fiddle with his reddened ears for a while before slightly averting his gaze.
“…I got it.”
“Mm-hmm.”
“Ah, I just mean I understand, so don’t take it too deeply. Okay?”
“…?”
There wasn’t really anything to take deeply… Yeo-woon’s lips twitched briefly before he smiled and nodded.

Really
I could go on about how this is fantastically crafted - albeit somewhat confusing at times - but man,
this is horror. this isn't romance.
It's a cosmic reincarnation horror about stalking, scheming, and knife-twisting obsession from a monster, slowly creeping in and taking over their target after their initial attempt failed.
The ML is the villain.
I feel sick to my stomach.
I'm gonna buy the officials if/when they come.
she's the most believable child I've ever seen in all of manwha lmao