beta
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