MCxKim have been together longer than that. MC long past left college...has ha a career and everything. When Keyla and Zeke got together....Zeke was away himself from his/MC's hometown. So when they got together/started dating, MC knew nothing about it.
They eventually 'come back' to where the MC is in college (they are all in the same college now)...they Keyla
cheats with MC...they stop...MC fools around with Kimberly, eventually starts dating Da-Som.
So Keyla cheated very early into her relationship with Zeke. At this point, we don't even know if they were 'exclusive' or not. If they weren't...then techincally no, they did nothing wrong (well, MC a bit of an ass but no, no one cheated if they were not exclusive).
Elduriel Yes...MC was 'with' Kimberly right after stopping things with Keyla MKII and before he started 'dating' Da-Som (who he already knew).
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At this point, I would not actually be surprised if Zeke knew about the 'affair' (let's call it that as said above, we don't know if Zeke/Keyla were exclusive or not) and I think Zeke always knew (as soon as they returned) that Keyla was in love with the MC and that Zeke would never truly 'have her' himself. I think this perhapos caused Zekes wanton behaviour all through their marriage (and that he proposed to not lose her to the MC..basically a pathetic simp).