SPOILER (can't understand full hangul so based from what I understood)
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Long story short, and to start with, the company where the MC and ML works does not allow dating between co-workers, and the 3rd person who the ML was supposedly cheating with knows this, was interested in the ML, and blackmailed ML to date him or else he'd tell and ruin MC and ML's career and out them in the process, now them dating had no indications that ML did the deed with the snake, more of just dinner dates and sweet talking. The ring was supposed to be for the MC when the ML was trying to clear things up and propose once things calmed down and cleared out but was stolen by the snake thus causing mishaps to happen while they're also mid-fighting every now and then coz of stress from work. Fast forward to the reset of 5 years to the past, things flow differently at this point cause MC handles stuff differently now compared to how he handled managing his people before (example: why they went to a company trip now due to great news and why they didn't before). And things progress, MC falls in love again with ML and vice versa, this time better than before, but with still miscommunications (seems to be the drive of this story), snake pops in again, tries to sabotage the relationship like how he did in the previous life, but this time it doesn't work, ending with the MC and ML living happily ever after.
Reading the raws... I dunno if I like or don't like the ML, I dunno if I could label him a cheater as well...